As cmptrdewd stated, the lion did in fact have it's mouth open. But if you look at the slomo part of the clip, just before the lion starts his jump into the guy in the foreground, the lion takes a hit to the jaw. Its pretty clear that the lion is seriously wounded, even before he makes his mad dash at the shooter. If that were a cat in his full faculties, that guy would have been toast. Just as it is, the lion got one paw on the guys arm to knock him over (he even loses his rifle). Looking at the video a couple of times shows me that this guide was a complete idiot. They were lucky that day.
TheRev