The best thing about this is that while I don't know any of these guys myself a lot of people I know do, and one of them was married to a coworker of my ex and by all accounts they're the sweetest folks you could hope to meet, of course.
it happens off screen, but it seems like Seth was being Seth and punched a girl in the crowd while flailing around. He's not GG Allin, so it probably wasn't on purpose, but his flippant attitude (kind of "What did you expect") and AC's general style seems to have set the crowd off as much as the original punch itself. It probably could have been diffused with an apology. Not to generalize to much, but crowds in San Fransisco are a bit different than what they were used to on the East Coast.