If your glue bond is not strong enough, then the tire rolls off the rim and you crash. This usually happens At high speed when you are changing your line. The added G force as well as the sideways force from the direction change put huge forces on the tire, causing it to roll off.
It is rare to be able to recover from a rolled tubular and ride it out. I've also seen a front tubular roll off and get caught around a riders foot. That was a nasty crash.