What probably happened was this: When the first shell "misfired," its wad got stuck in the barrel, and when the shot from the second shell ran into it, the pressure bulged the barrel. That barrel is now unsafe to shoot and has to be replaced. You may also have noticed an odd whispering noise when you fired the second shot. That was the wings of the Angel of Death. He gave you guys a pass that day. Blowing up a shotgun barrel can cost you a hand, or your eyes, or your life. Always check the bore for obstructions when you have a misfire.