"...these cases have been reloaded many times..." That'll do it. Like kcw says, it's work hardened brass. Pitch the cracked cases and anneal the rest.
Annealing is easy and costs little. Use a regular propane torch(kit runs about $14 to $20 in Home Depot) and a pan of plain tap water to the shoulder. Heat the case necks and shoulder until the brass visually changes colour and tip 'em over.
Anneal is not part of the regular reloading process. Only needs doing when you get a cracked case mouth.
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