Must be from the nipple end. With rough handling, a slightly loose cap can fall off the nipple - it happens. If it happens, then it is possible, however remotely, for a spark to travel down the cap-less nipple, past the choke at the bottom, to the powder charge and ignite it. That is a long way for a spark or sparks to travel, all the while cooling down. But - not only is it remotely possible, it has happened to the detriment of whomever is holding the rifle.
Long cuffs on substantial gloves would help.

Arc Welding gloves today are quite similar to those "Cavalry Gloves" of yesteryear. Still, nice to look at, but not for me to fire.