Artificially generated Peak Oil event.

One of the trickiest technological approaches to problem solving, and the one that has consistently generated the bulk of our unanticipated consequences, has been the introduction of biological controls into the ecosystem. That said, we continue to develop such controls because they tend to be quite effective, at least in the short term.

So; enter Alcanivorax borkumensis, a bacteria that literally eats oil and has been designed to help with environmental contamination scenarios when oil tankers & rigs spill massive amounts of oil into the ecosystem.

We are now working on a transition away from fossil fuels globally but setting up new infrastructure, bringing costs of new technology down etc. takes time and the world isn't ready for an instant peak oil event.

But, if this bacterium got out into the ecosystem in an uncontrolled manner and infected all the oil reserves globally at the same time, we could end up with oil depletions on a massive scale within months.

Believe it or not, this would be every bit as devastating as a human infection. For a start, your 75% death rate would be achieved relatively quickly by virtue of the lack of food getting into large cities. Oil (read as petrol) is now a critical part of the food chain because of the food distribution requirements of larger population centres. Large cities simply don't produce much food, but they consume massive amounts of it. Disrupt fuel supplies for a month, and you'll face a complete and total breakdown of law and order and city life, triggering massive unrest, deaths from starvation and civil disobedience, and a mass exodus from the cities into country locatinos without the infrastructure to support the survivors. Not to mention that most farms today are 'productive' thanks to fuel and technology that is no longer available.

Also, many modern farms contain genetically modified seed stocks that can't be replenished through 'seed cropping' and the fertiliser needs of the soil are massive, meaning that without a functioning distribution network the farms simply couldn't support the populations coming out of the cities even if they wanted to - farming looks too different from 100 years ago when all those people could have been put to work on the farms instead of machines.

This is not a total extinction event by any means, and certainly pockets of civilisation would survive because of the use of nuclear and renewable electrical grids and the uptake in certain areas of electric vehicles, but planes would be grounded, we don't really have a functioning electric heavy vehicle infrastructure in place and many societies still rely on petrol as a key driver of their infrastructure. As such, you'll find that your society would face massive losses in the short term, and significant losses over the medium term, if all the oil on earth just suddenly vanished.

Here are a few:

1. Super massive solar flare removes most of our atmosphere/scorches the earth. Hard to balance between everything destroyed and nothing destroyed, the earths magnetic field stops most solar wind from doing damage, but it could be overwhelmed.




2. Cult like religion takes over with a goal of depopulation. No explanation needed.




3. Collapse of an overly centralized system. Say technology advances to the point of completely replacing human labour. To counteract the massive loss of jobs, the government takes control of production. If the government was then to collapse, the supply of everything could disappear. Even if the production mechanism is fixed, the complete disappearance of the food supply for example would do massive damage in just a week (though a couple months would be where the real collapse would probably peak).




4. Widespread use of a flawed genetic procedure. Say a gene editing program begins to give everyone a gene that prevents cancer. It turns out the gene is flawed and stops the ability to reproduce after a number of generations. It is likely that technology would also then be able to remove the gene, but this can be an odd one.

In the webcomic Nine Planets Without Intelligent Life, humanity just felt in such an hedonic state that people stopped reproducing. They would have robots serving and pleasing them in all ways possible and lost interest in human reproduction. Eventually humans all died out.

This is tangentially aligned with the V.H.E.M.T., which is a thing in the real world:

The Voluntary Human Extinction Movement (VHEMT) is an environmental movement that calls for all people to abstain from reproduction to cause the gradual voluntary extinction of humankind.

A near-earth supernova could do the trick, as could a neutron star flare.



Mass sterility induced as a side-effect of a commonly used drug or energy drink. Something like this is in fact already happening.



Most people choose virtual reality over real life and neglect procreation until too late. People used to physically perfect (and willing) virtual partners may be disgusted by real-life sex partners or decide that it isn't worth the hassle. This is already happening in Japan.



The successor to the Large Hadron Collider; the Future Circular Reactor, may accidentally produce a black hole that absorbs the earth or even cause some totally unforeseen event, like creating a short-lived wormhole that sucks in most of Europe and causes massive earthquakes and tsunamis on the rest of the planet.

Earth impact events

Most people think of catastrophic asteroid impacts to be extremely rare events, or don't even consider them plausible. A category 9 or 10 impact on the Torino scale would cause extreme damage:

9: A collision is certain, capable of causing unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.

10: A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

Quantum tunneling

Just for fun, let's take out the "relatively unlikely" clause from your requirements...

Quantum tunneling (a real-life version of wall glitching in video games) can, in theory, cause all of Earth to spontaneously collapse in on itself and become a black hole, but the chances that it would occur before every last star in the universe has become a black dwarf is likely smaller than the chance of you winning every lottery in the world for the rest of your life, and not being accused of cheating. This event is physically possible because subatomic particles do not exist in any one, discrete point. They are actually probability functions and exist everywhere with a specific probability. This allows one subatomic particle to pass through a barrier that it otherwise could not pass through, such as another subatomic particle. If enough particles on Earth happened to pass through each other and end up at the same place, the mass would be sufficient to cause an immediate collapse into a singularity.

This is actually something that we expect to happen in the future, at least if protons do not decay first. Between around $10^10^26$ to $10^10^76$ years in the future, all objects larger than the Planck mass will see this fate. From a Wikipedia article on a timeline of the far future, this is:

[...] the time until all objects exceeding the Planck mass collapse via quantum tunnelling into black holes, assuming no proton decay or virtual black holes. On this vast timescale, even ultra-stable iron stars are destroyed by quantum tunnelling events.

An additional note is given regarding the estimates of the timespan:

Although listed in years for convenience, the numbers beyond this point are so vast that their digits would remain unchanged regardless of which conventional units they were listed in, be they nanoseconds or star lifespans.

A False Vacuum Collapse

The quantum vacuum is a measure of the lowest possible quantum energy state. Some scientists have hypothesized that if our current quantum vacuum was not the actual lowest energy state, but simply a metastable local minimum (a false vacuum), then it would theoretically be possible for the universe to transition into the actual lowest energy state, which would release a very large amount of energy from basically everything.

A false vacuum collapse, could if it occurred, destroy all of the observable universe, starting from a single point and expanding in a bubble that expands at the speed of light. Since it expands at the speed of light, we would have no idea that the wavefront was approaching us until it reached us. Once that occured, humanity, the Earth, and the Solar System in general would all cease to exist as space-time exploded in a massive release of energy.

I'm surprised no one brought up super volcanoes... One of them blowing would bring upon the world volcanic winter (and a dinosaur-like extinction event)

The eruption itself might or might not be preceded by earthquakes and deformations of the landscape surrounding the super volcano itself, announcing the imminent explosion.

Once the blast occurs, the first damage would be to the surrounding area by means of blast-wave, hot ash and thrown debris from inside the
volcano.

The blast itself will throw HUGE amounts of ash and very small particulates into the atmosphere, preventing the sun's heat and light from reaching the ground. This would, in turn, cause a rapid drop in temperature (volcanic winter), and would cause crops to fail and livestock to die due to decreased temperatures and lack of sunlight.

There is a very interesting paper written in 2014 which has tried modelling the Yellowstone Supervolcano eruption fallout. It details the ash distribution (over US) taking into account the wind patterns. You can find it here. (I am not connected to that study in any way)

Edited for a more complete answer

Information Collapse

Human Society is dependent on data. While it seems trivial to run the entire world "by hand", it's not possible on the scale humanity has found itself.

Imagine, if you will, a malware that would infect every remotely sophisticated information processing machine. Servers, Personal Computers, Laptops, Smartphones, Embedded Devices, Calculators, etc. All of them would cease to function.

Society as we knew it would collapse. We wouldn't be able to manage traffic, bring goods from A to B or instantly communicate over long distances. This may seem benign at first, but let's have a look at things.

Timeline of events

At the collapse

People would notice that their phones and computers would stop working. Some would angrily shout at their headset why they got disconnected from Counter Strike when they only needed to win one more round to win the match. Others would try to smack their remote because the TV wouldn't turn on.

Many would then try to gather information about why nothing is working, but they can't "google the problem" because Google itself is no more.

Shortly after the collapse

People gather outside, trying to figure out what's going on. Power has gone out, because the control software for the power plants has gone out. Some are quick to realize something is amiss and try to gather as much food as possible. Supermarkets can't make bills for the customers, so they just take food by force. What are they going to do? Make smoke signs to call for the police?

Several days after the collapse

People with older cars might have a chance to get them running, at least until the gas is out, but most people will favor bicycles or similar modes of transportation. Communication between people is now mostly verbal or with notes left behind. People try to find their loved ones to make sure those are okay. Food supplies are still fine, but decreasing. Fresh water has been identified as a valuable commodity.

Several weeks after the collapse

Many people have died of dehydration, others of the violence that has resulted from the lawlessness. Communication has become difficult, especially for authorities to coordinate each other. Remote areas either died out because of the lack of supplies, or managed to sustain themselves due to access to water and food.

Several months after the collapse

Technology has somewhat revamped to pre-WWI era. Many people have died due to the lack of supplies and coordination among people, but far more due to mass hysteria among people.
Military forces either aided the government in keeping the leadership, or took the opportunity to become leadership.

Several years after the collapse

Humanity has stabilized again. Some areas recovered back into civilizations, while others are better described as barbaric and lead by whoever has the most force. The collapse mostly affected the developed world, especially overcrowded cities such as New York, Tokyo, Hongkong, Paris or Berlin.

All very good answers. I will, however, tackle the problem with oblique approach:

1. Aforementioned Solar flare - it doesn't have to be big. All it needs to kill in excess of 75% of the population is to coincide with magnetic field reversal. There are, actually, some achaeological finds that suggest something like this happened on Earth already. In fact, some suggest just magnetic pole reversal can do it - predicted result would be dramatic increase in harmful solar radiation that's reaching Earth's surface in many places, increasing temperatures and killing with prolonged exposure.


2. Collapse of electric grid world wide (for any of the various reasons) - most of the urban population does not stock food, average household has less than a week's worth. Stores do not have more, too - at most 3-4 day supply. Once electricity is out, logistics and refrigeration is out, causing civil unrest. It is estimated that without power for more than a two weeks (maybe even one), in about 6 months Developed countries' population would drop by at least 75%. But obviously it would not impact undeveloped areas of the world that much. Africa and similar would probably see 50% drop.


3. Supervolcano or several large volcanic explosions in a year, at the right time of year - they would increase occlusion of the atmosphere, reducing available sunlight and reducing global temperatures, which in turn will impact crop yields. Not sure how big taking into account current high CO2 content (it is a plant nutrient, so the more the better for them), but just 10% reduction means famine, which means social collapse worldwide. and then see No 2.

A celestial body passes through the inner Solar System

Anything ranging from a planet to a black hole moves close enough to perturbate orbits of planets or asteroids.

Possible outcomes are:

• Impact event.


• Tidal forces shatter the Earth's crust, severity varies from earthquakes to seas of magma.


• The Earth's orbit changed or it is ejected from the Solar System. It becomes really hot or cold in several months after the event.


• Orbits of the Earth and some asteroids from the belt now intersect. Delay of the impact may be anything from months to thousands of years.


• Another inner planet shattered with its pieces gone astray.

Well, this has been partly mentioned in one form or another, but a powerful solar storm could trigger a mass extinction of humanity (and only us).

I haven't seen it in any apocalyptic movie or book yet, but something like this happened before : https://en.wikipedia.org/wiki/Solar_storm_of_1859

TLDR: a powerful coronal mass ejection from the sun hit the Earth. It was seen as auroras all accross the continents and created one of the strongest geomagnetic storms on record, causing telegraphs (the only electrical systems at the time) to fail. If it happened nowadays, replace telegraphs with most of our electrical grid and electronic devices (that means no cars, trains, planes or boats too).

From there, the minimum we can have would be mass deaths as cities stop getting supplied in food and water and possibly the complete breakdown of society, replaced with an 'everyone for themselves' scenario.

Humanity won't go extinct, no risk of that and wildlife will be unaffected.

Some other fun ones:

If we humans really are a simulation, some higher power entity might get bored and input something ridiculous onto our planet, like a new life-form, to kill us all off.

Somehow, a deadly poison gets into the oceans, killing a lot of people who drink/come into contact with it.

On Earth, some government-type people decide that cannibalism would be the best solution to both world hunger and overpopulation. Enough said.

Self fulfilling prophecy: The scientists lie to the people, convincing them that an unrelated extinction event really is happening, leading to massive "drink-the-kool-aid-style" suicide events.

A new kind of addiction ravages the world, decimating society at large.

An advanced society decides to kill off us pesky humans to take over our planet. Instead of actual warfare, they just put a spaceship into orbit around the sun, permanently eclipsing us. A couple of years later, and we humans, having no real recourse, are mostly wiped out due to lack of sunlight.