Originally Posted by BlazingPedals

Improper tire mounting. I'm guessing the tire was recently mounted on the rim, asy within the last week or so. Tube was stuck under bead, and eventually blew out. This would not happen if the tire/tube were mounted properly; it is the tire's job to withstand the internal pressure, the tube only has to seal the inside of the tire.

edit: As I think about it, some tire/rim combinations fit so loosely that this can happen despite the best efforts to the contrary. If the tire is so loose on the rim that you can't get it to seat properly all the way around, get a different brand tire.