Orcas are very picky eaters. Particular groups might specialize in eating one thing like fish and avoid eating other things even to the point of starving, while other Orca pods that pass through the same area will mainly eat marine mammals.

Thing is, at some point in history, a trend-setter for the pod chose to eat that one thing and teach that to the pod...

Right now, no Oracas eat humans or even attack humans, but once a trend-setter decides humans might be a good snack, and she tells the rest of her pod... Well, the ocean will be a more frightening place.

They've already taken the first step in that direction.