[user104658]

Blanks can kill if there is debris in the gun, e.g. a stone lodged in the barrel will fire out like a bullet if a blank round is fired. A blank is just a regular round without the projectile (the bullet), so add any projectile to the mix… and that blank is effectively live.

So could have been that, if not an error made when loading the gun. The other possibility of course is deliberate sabotage.