A friend of mine who was there told me that the puncher hit the tripping guy because the tripping guy was trying to choke his friend. Find out the story when it hits yahoo or msn online tomorrow I guess.
Doesn't sound implausible, as you can hear someone tell him "you're in a theatre with people", trying to get the guy back to reality in some way.
Ok, just read this on youtube comments. (I can't believe I'm reposting comments from youtube!):
"Deal gently? You have no idea how bad it was. You can't tell from the video what was happening. I was a row behind. The guy manhandled a kid and hit people. I doubt this was acid- more like meth. The police tazed him 5x outside & he was still going. Dude who punched him did it to stop him from strangling a man- & it worked."
If true, it does seem like reasonable force to punch the twat. After his subsequent journey, I doubt he came to in Louis XVI-style police cell.