I've had two accidental discharges (of a firearm lol) in my life. One was an worn out trigger on an old breach loading revolver (loved that thing) and the second was not unlike what happened here. Someone else had handled the firearm and I subsequently went to clean it. Instead of checking because I assumed it was clear (one in chamber, no magazine was present) I thought I was going to dry fire it and instead fired off a round. At least I was aiming upward, nobody else present
but still. As mentioned, no matter what you think, always, ALWAYS check it first.
It sounds like he fired more than once, which suggests he thought he was firing blanks.