Maybe after the worldstone was destroyed (maybe it wasn't it could return who knows) and the soulstones Hell finally was able to invade heaven and kill Tyrael's kind? Would make sense that the demons would take out the heavens before the mortal world because the heavens are more of a threat. Except of course for that one mortal (the one you play) that pwns the entire army and it's leaders with a sword.

That being said I wanna see the downfall of the heavens.

Heaven + Hell in Diablo = Epic movie scene.