Ruptured HXP case

Would like to know what caused this?

Our local club bolt gun match. Using Rock Island 1903, slow fire prone. HXP dated 1970 ammunition obtained from CMP. Had already fired several rounds. Squeeze trigger, rifle fires, normal recoil, immediate burning to my face below shooting glasses. Eject and look at spent cartridge. Jagged split starting about eighth of an inch in front of extractor groove going down cartridge side towards the shoulder for about an inch.
Good primer strike. Fired primer looks normal.

Haved fired alot of this ammunition over the past year. Both Garand and 03. No problems. Did notice after resuming that several rounds from this box were hard to chamber and/or eject. All clean, bright brass.

Rifle head space, bolt, bore, etc is good. Hatcher hole. Have been shooting this rifle for many years.

Sorry no pictures.

Thanks for any help,
bonnie

Save the damaged casing, the box, etc. and contact CMP ASAP w/ lot number and any further information that you can provide. The split you describe is dangerous. CMP needs to know so that any necessary steps can be taken. Sincerely. BruceV.