Well, based entirely on my limited knowledge of the drill on our WW2 twin-barrelled gun of roughly similar calibre; doofus is making two main mistakes:
you don’t load a piece while standing directly behind (i.e., right in the path of the recoil should there be a premature discharge);
a round should be pushed in briskly, with the hand in the form of a tight fist moving from bottom to top, not poking around in the breech area with loose fingers–that’s asking for Darwinized ablation of the digits.
I’ll take a WAG here that either there was a problem with the protrusion of the striker, and that when the breech snapped shut, the striker hit the primer of the cartridge; or that there was a problem with a spring mechanism that holds back the striker–if this failed when the breech closed, and allowed the striker forward, the cartridge would have been fired.
Lastly, someone might have had their foot or hand on the trigger mechanism (if there is one for this particular piece), and had it activated when the breech closed.
In any case; investigation ensues–whoever was the instructor or in charge of the shoot is in trouble over this one.