If it happens with reloads, it could be a large chamber. Stretching and resizing can cause the molded in hull to separate from the brass head. Not common really, but does happen. The only shells I have ever encountered do this regularly were the Winchester AA of 1970's vintage. Separations were common with then, especially with slide action shotguns. Caused a real problem around trap fields at the time. Everybody on the line carried a wad knocker in their pocket to drop on the hull to get it out of the chamber.
I have avoided AA since then due to the experience. While I was later shooting a lot of sporting, I gravitated to Activ for a long time. Some people didn't but I loved the all plastic hull. No separations and they seemed to me to be extremely strong, with no rim problems and their capacity seemed to give moderate pressures. If memory serves, I never resized and they worked great. Best thing was that they had some really different types of loads. I still have a few boxes of their spreader loads. I think they actually started that trend before some of the specialty brands.
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