I something similar a few years ago. It appears that the case head flowed back into the ejector hole. It's called a "flashover". This is the result of a light powder charge where the powder lays throughout the case sort of like laying a 2 liter soda bottle on its side with one liter of fluid. The theory is that when the primer is ignited it does not burn from the back of the case forward but ignites all of the power at once causing an overpressure.