Could the primer have not been completely seated? Sitting proud and crushed by the bolt? Could a piece of debris got between primer and bolt? Could the pin have become jammed in the forward position, possibly by some debris in its channel?
I once knew a guy shot several times through the hand by his own 1911 going full auto. Turned out the pin got jammed forward by a tiny pice of steel that got loose from the spring. When he cycled in the first round it went full auto on him.
If possible, a close examination of the primer from the exploded round. It would have been blown back, but does it have a pin dent in it?