Reasons for the Apocalypse:
Mages casting massive fireball or lightning storm spells that wipe out all the major cities.
Wild magic runs amok.
Magic leaves the world, even if only temporarily.
Spellcasters all go insane.
The orcs and goblins win!
Dragons go on a lemming-like burn and eat frenzy.
A powerful artifact of evil is used by a psychotic suicidal person.
The forces of good (angels, etc.) all leave the world for some reason.
The rat-men of the sewers come out at night and kill millions of people.
Lycanthropy runs rampant, becoming even easier to catch for some reason.