[Unclenick]

Primers back out during firing because their brisance (suddenness of explosion) is high enough to drive the primer cup back faster than their gas can escape through the flash hole to relieve the pressure. This is the reason drilling the flash hole out prevents it from occurring. You can also make a load with light bullets and too little powder that doesn't sustain pressure high enough for long enough to drive the case head back enough to reseat the primer.