According to an LA Times article from the day after:
The disturbance broke out about 2 p.m. behind bleachers being used for the final day of the Ocean Pacific Pro Surfing Championships, which drew a crowd estimated at 100,000 people.
Witnesses said the melee had no direct connection to the surfing contest but instead was triggered by two or more men behind the bleachers immediately south of the Huntington Beach Pier who were trying to take off the bathing suits of two young women.
Bro. Bro. Brah. This does not sound chill at all.
Those who were injured, including the six law enforcement officers, reportedly were hit by thrown objects or struck by people in the rampaging crowd. Most of the injuries consisted of sprains and head cuts.
No tear gas was used, and the only gunshot was a warning shot fired by a lifeguard.
Check out the full set here — there are a lot more.