Reloading Questions

I have an odd situation going on with my dreaded .223/5.56 Mossberg MVP. I just tried a new powder and got great ES and SD numbers and alright accuracy with my bullet combination. Anywho, a new phenomenon has reared its head. The fired brass is pushing out the primers. Like the primers are un-seating themselves. It's not over pressure (primers crater and/or punch a hole in them) and all the primers have a good fit and aren't loose by any means. What could allow the primer to push out like that?

I have a theory that the brass isn't actually headspacing properly and the firing pin is pushing the cartridge farther into the chamber making a gap with the brass and the bolt face...