Note: the below is a big honkin' setting riff based on one of the new D&D races in the PHB2. If that's not to your interest, please accept my apologies. You can totally stop reading now. :-)
So, I dig the devas from the PHB2. Former angels locked into perpetually reincarnating mortal forms, with each incarnation retaining some memories of their previous lives? Righteous.
(No pun intended. Well, OK, maybe a little.)
But in my campaign world, I don't think I'd have them be just any old former angels. No, the devas were all loyalists during Asmodeus's Rebellion, and their current condition began during that strife. But while the few memories and fragmented writings they have from those early years confirm this, no deva knows for sure why they all became mortals.
Some think it was Asmodeus' punishment for those former colleagues who didn't join his cause. Others think it was a desperate means of escape before their once-beautiful Astral Realm was turned into the Nine Hells. Still others think it was actually a first strike by Asmodeus, trapping hundreds of loyal angels in mortal form so that he would have a clear path to their doomed god. But none can remember; none know for sure.
What they do know is that answers are not forthcoming from the devils, for Asmodeus's enmity for the deva is so great that any devil caught consorting with a deva learns the true depths of Hell's torment. And even if a devil did answer, they'd probably lie.