Probably forgot he was the one with the 'real' gun
I can see it right now, looking around at all the other dipshits shooting the guy with thier paintball guns, in the excitement I'm sure he just wanted to do what they were doing. . . I can imagine it right now: "hey BillyBob that was a good one, check this out! (BANG!) uhhh. . . . . uh. . . oh shit." Maybe the guy tied to the tree was looking down at the time? In any case I suspect beer played a part in this as well.