It makes no sense.
Because they were hungry?
revenge!
They had it coming.
Orcas eat sharks, dolphins are related to Orcas ...you send one of mine to the hospital, we send one of yours to the morgue. It's really that simple.
Dolphins can usually beat up sharks.
Really?
The last I heard, yes. In fact, in Jaws 3D, I think a couple of dolphins help some humans escape an attack by distracting the shark.