Originally Posted by jepp2

All the gases from the primer burning must rush through the flash hole in the case. It generates enough pressure to back the primer out of the primer pocket.

This happens when you fire a loaded round also. But in that case the pressure from the burning powder forces the case backwards against the recoil shield, thus reseating the primer in the case.

You didn't have the reseating action by just firing the primer. Folks that shoot the reusable rubber bullets wind up drilling out the flash hole larger.

When I need to remove live primers, I slowly deprime them using a universal decapping die. I wear eye protection and gloves, and I have a piece of PVC pipe cut to slide over the cutout area of my Rock Chucker ram. I'm not suggesting you do this, but I haven't ever had a primer go off. But I am always prepared just in case.