It happened at a re-enactment, most likely was not swabbed or vent stalled properly and the aluminum foil wrapped cartridge tore when rammed home,
he was using a standard rammer and wearing gauntlet gloves, gun fired when charge was rammed home, some say gloves add safety,some believe
that they tend to grab too much and your hand never has a chance to release from the rammer when this sort of thing happens.
A French rammer or a Shepperd's crook rammer would aid in preventing such injury