Been there! Did you shoot a bunch of those loads? Is so, and it happened one time, I would say it was a bad round.
If it happens more then once! Bad gun.
Are you a handloader? If so, make some light loads and check those casings to see if they are getting swelled belts or any imperfection for that matter. If so, again; BAD GUN
I have heard of this happening a few times with people that reload and use a collet die.