Possibly the cocking piece became unscrewed allowing the firing pin to strike the primer. How else could it fire with the bolt open cocked position? Even if he was squeezing the trigger the bolt would be closed before the cocking piece pin could go home. The pin cannot pass the sear arm unless the trigger is pulled. The pin cant fire without the bolt being rotated. If the bolt is rotated closed then the bolt could not move rearward. Only other thing is there was a barrel obstruction. Since we cant see the pictures all we have is conjecture. Unscrew the BAS and check.