The pressure from the first cartridge (that separated the case and the barrels) probably produced enough recoil that the rifle doubled on its own. Or scared the guy so bad that he slapped the rear trigger in his moment of surprise. Either way, Curl is right: doubling will not wreck your rifle.

The intention of the post was remind us all to be very careful when using a filler of any type, or a reduced load, or when reloading in general.