The idea of going multiplanetary is not to abandon Earth out of necessity, but out of precaution. When you make a backup of your hard disk, it's usually not because you intend to go use your other one for target practice. Waiting to leave Earth when problems become clearly insurmountable is akin to waiting to backup your HDD until you notice it's failing. Indeed the very first thing we should start doing once we begin colonizing our second planet is planning to colonize the third.

There are countless ways human civilization, if not the human species, can come to a rather abrupt end: supervolcano explosion blotting out the sky, directed gamma ray burst destroying the atmosphere ( hypothesized as one of the reasons for the great ordovician extinction ), comet impact acting similar to the supervolcano, random evolution creating a supervirus, and so on. And the countless ways we might manage to kill ourselves go without saying: nuclear war, nukes, deploying weaponized viruses, even far more innocuous things like fertility < 2.5 for too long.

Many of these causes can, have, and will happen abruptly.