Mike,

My only guess would be from a physical standpoint, which would relate to compression analysis.

Would seem it most likely occurred due to tight constriction, as the shot was actually reshaped, which would take 1) intense density coupled with, 2) intense pressure. The only area of the explosion that would compensate for this is the choke, unless that particular shell was "overloaded". If you have more shells from that box, and have access to a scale, try to weigh each shell and check the tolerance.

But that's just a biologist's best educated guess...