I think I've posted this story before.
I was hunting with a guy who was using reloads. He shot them a lot at targets with never a problem. He had a few squib rounds while hunting though. The problem was the wads were slightly to small for the casings. The shells bouncing around in his game bag let some of the powder sift to the upper side of the wad. Causing a problem similar to what is shown in the photo.
terc