Having been to two high school graduations, the first of which was as a "volunteer" usher/"security" person, I can attest that these events bring out the worst in people. It's actually really shocking just how much people turn into assholes. I'd guess it's the limited seating, the pseudo stress (of a pseudo important event), and poor planing on the part of the attendees, that contribute to people becoming complete assholes.
It was such a miserable thing that after that first "volunteer" gig, I told my wife (who works for the school) that I would never do it again. The only reason for the second graduation attendance was because it was our oldest son's. Once the other two kids are done, I'll be happy to never attend another one of these things again.