I think the brass may have gotten brittle.

by CJM , United States, Thursday, February 19, 2015, 00:43 (3570 days ago) @ Hoot

If the brass is the same age as the gun, could that be caused by mercuric primers? Not normal work-hardening or the brass would have come apart during sizing. I don't think brass normally can get harder and brittle by just sitting, but mercury contamination would get worse as the mercury continued to dissolve into the metal. It might be a good idea to see how brittle the case heads are after annealing, you don't want to find out that they are still brittle by firing them in the gun.