Unless he slammed the hell out of the action, closing it should not have enough energy contacting the primer to set it off. Just pressing the primer isn't what makes them go bang. It takes a pretty good directed amount of energy to set off a primer. If all it took was pressing hard on a primer to set it off, they wouldn't be anywhere near safe to ship or carry. I would say there was something else going on like he goofed and had his hammer back or something along those lines.