The New AA hulls will sometimes develop a crease or wrinkle just above the top edge of the basewad. It will weaken the hull during subsequent firings. If you see the crease or wrinkle, it's best to toss the hull. There are some adjustments that can be made to eliminate it. Contact your press manufacturer for help getting it set.
I've never seen one separate that way, but I also don't load hulls with certain defects.