There is an extended vid somewhere of that where they found that there was a casing stuck inside the action somewhere. It was a longer vid, and it showed that the rifle was functioning for a long time with it inside.
I am trying to find it on Youtube, but I swear I've seen it.
I've had that happen. Not with an AK, but an FN. Never really did figure out what happened. Whatever it was, it fired out of battery on the last round. What was left of the brass was lying on the follower and the bullet was stuck in the barrel just forward of the throat. I caught a lot of little bits of brass in the face. The next day I looked like I had a bad case of acne.