I believe the round responsible was an 80s vintage SLAP (Saboted Light Armor Penetrator) roundFrom the pictures the round was quite full. Maybe that's normal maybe not. If normal then maybe KB's issue was it was actually loaded with the wrong powder. Or maybe too much of a powder that doesn't take a full case to work. Double charge possibly. Possibilities are endless. Thanks for the pictures btw!
It could simply be that less than ideal storage conditions altered the burn characteristics of the powder of that particular cartridge.