Malibu Lagoon State Beach
On first impression one might attribute this confrontation to territoriality, competition for food, some kind of courting joust or ritual or maybe just establishing a pecking order. After observing for a while, however, I'll go with the more anthropomorphic explanation that the bird on the right is just a jerk. He was going from bird to bird, "attacking" them without apparent provocation, didn't seem to care if the victims left the immediate area or not.
Member of the Flickr Bird Brigade
Activists for birds and wildlife