Stuff like this happens, sadly.
Had a dude in Infantry Squad/Team live-fire certification get shot in the ass back about a decade or so ago. It normally comes from inexperience, or in this case, complacency and lack of secondary checks.
ETA: There are Ammunition Point procedures and regulations that must be adhered to for safe operations, but running the Ammo Point at a range tends to be a detail not treated particularly serious by those assigned to it. Usually, it turns into a exercise of hand receipt control and not the proper control and safety of ammunition distribution.