I once encountered several hundred .30 M1 Carbine Lake City fired cases that had that problem. On decapping, a large percentage of the primer cups would pull apart leaving the edges stuck in the primer pocket. On examination, the insides of many of the cases were corroded, apparently from being rinsed in case cleaning chemical and being left to dry out. The case corrosion seemed to deteriorate the inside of the primer cups making them separate. Taught me to closely inspect when buying cases in bulk.