I have had this happen to me about 4 or 5 times whenever the new AA HS hulls came out and as a result I do not reload the AA HS hulls anymore. I stick with the Remington STS or the older AA hulls when I reload.
I had that happen when reloading a Nitro 27 hull about two weeks ago. The brass head just came off after the shell was crimped. No Cracks or deformities in the head ( of the shell, maybe a few in mine) and none of the other shells that I loaded before or after could be pulled off by hand. I guess stuff happens. The first time this has happened to me in almost 25 years of reloading.