Hosehead Jones

I have a PF9 that I thought was key-holing at the local indoor range. Then it struck me that it was flapping the target sheet around. The taget is only held by a clip at the top. I added a sheet of cardboard to back it and voila, no more apparent keyholes. After adding a backing, I've only had holes. No, nothing else I shot does this. I have no real explanation for the phenomenon.