I've often wondered about this too, oddly enough. Maybe it's a torque induced roll, as you depart terra firma your front wheels get in the air first, leaving the only point of contact your rear wheeles, and while you're firmly holding down the accelerator, this induces the back flip briefly before they also become airborne?
I've flipped a Jeep and had it land vertically on its grill and stick there. That worth anything? 
Faeplant.
