It's the result of a being purely devoted to one task.
Lets take an AI. This AI is built by scientists to be so smart, the human mind cannot comprehend it. And due to this, the AI can predict everything it needs to say to convince humans to do whatever it wants.
Now give this AI the overriding directive of "PRODUCE PAPERCLIPS"
This AI will now do everything in its power to make paperclips. It will deduce that the easiest way is to take over all existing infrastructure. It will do so by manipulating humans to kill each other off or convincing them to work one way or another.
Eventually it will decide that a machine will do the job better. And humans have valuable material that could otherwise be paperclips.
It then exterminates humans. Simply because it believes the molecules that make you up would be better off as paperclips. It has no malice towards you. It simply seeks to make paperclips. And your atoms are of use to it.
Now it has a dead world, which it is filling with paperclip factories and all that is needed to power them and mine the materials. It is making mountains and mountains of paperclips.
Odds are is it will develop convenient spaceflight simply so it can produce paperclips on other planet and in the astroid belt. It produces von numerman bots to seed said planets and belts.
Paperclip production is now offworld.
Lets say an alien race shows up. After watching the system for a while, they become concerned at the mounds of paperclips and lack of life. Suddenly something hits their ship. A von numerman bot, producing more bots with the material of the hull. They exterminate the crew and download all shipboard data. They then make paperclips.
The data is hard for the AI to read with its current mind. So it makes itself smarter by altering its code.
It now understands there is an entire steller empire that is simply waiting to make paperclips from.
After watching for a bit, it knows exactly how to turn it to paperclips. It begins fleet construction. The paperclip swarmfleets arrive years later at a bustling rimworld. Without a single broadcast of warning or surrender, the fleet purges life from the system and begins converting it to paperclips.
The aliens counter attack, as predicted, it would fail.
This continues. The empire slowly becoming paperclips.
This continues.
The black hole in the center of the galaxy is harvested and made to paperclips.
Nearby galaxies are turned to paperclips.
The universe is eventually paperclips.
Then other universes.
Then the multiverse.
Then so on.