I decided to go with this for BL-opoly since it was the one sci-fi book I currently had checked out from the library. Very first page tells how the apocalypse came about: First a comet appeared in the sky that caused all that saw it to go blind. Then a virus from the comet started wiping out the population and turning the dead into zombies. Then the survivors decided to build an AI to help survive, but it decided to save the planet by nuclear fire. Then aliens invaded. But they ended up fleeing when natural disasters kept wreaking havoc on the planet. And that's how the apocalypse went down.
I like how, instead of going with just one reason for the apocalypse, the author threw in all the reasons. I just kept laughing when it continued to escalate and then aliens suddenly appeared and just as suddenly decided they didn't want the planet after all since it was such a disaster.