Humpty Dumpty;
I think the primary cause of burst bbls is an obstructed bore. Following that I would be inclined to take your Dad's Friend's opinion as the next most common. Actual Pressure Bursts, ie the pressure of the shell was too high for an unflawed bbl, even including damascus bbls being fired with modern shells, are extremely "Rare".

Miller/TN
I Didn't Say Everything I Said, Yogi Berra