There is a theory I read that said that it was indeed a boat that accidentally killed a female's infant. She attacked and figured out that hitting the rudder would make them stop and then she proceeded to teach the pod.
Another I read was that it started as juvenile Orcas just fucking about and now it's turned into a preferred pastime. AKA, Orca hooligans.
Both seem anecdotal and not scientifically provable though.