There was rioting at this contest in 1986. It used to be held on Labor Day weekend, but the city moved the contest to the end of July, hoping to avoid further alcohol-precipitated rioting.
I took my youngest boy down there a couple of years ago to check out the skateboarding, but they had a some big free concert this year and the crowds are obviously out of control.
See the Los Angeles Times, "Huntington Beach riot: bike shop workers fought off looters."
Check the video at that link, and more at Legal Insurrection, "Huntington Beach Rioting Follows U.S. Open of Surfing Competition."
And checking YouTube:
More here, "Huntington Beach Riot 2013 #1"