Detonation is a very plausible cause here. Fast powders and low case density are a dangerous thing in rifle calibers.  Too large a diameter of projectile could also cause this.

A note about the Brits.  There's a story going around the FAA about the Bits.  They were looking for  a way to test the windshields for their high speed trains.  They asked the FAA to borrow the "Chicken Gun" that we use to test aircraft windshields.  It actually shoots chicken carcasses at the windshield. Well, the Brits, using the Chicken Gun, busted just about every windshield the had and sent a report to the FAA for review.  They couldn't understand why all their windshields were breaking.  After the FAA read the report, they sent the Brits one recommendation.  "Thaw out the chickens before you shoot them".