They've found out that it's fun. as is not unusual in predator behavior.
ORCA 1: Check this out! (Swims directly at a sailboat, then turns to sideswipe the boat and breaks the rudder off)
ORCA 2: Ahhhahahahahahaha! Did you see them run around like seals with their heads bitten off? They looked like their hair was on fire and their asses were catching!
ORCA 3: Do it again! Do it again!
ORCA 4: Okay, let's cool it here. Do you want humans to begin coming at us with harpoons they way they do with the Minke Whales? They leave us alone because we don't offer them any good commercial reason to hunt us, and we don't eat them, even though we could, as bad as they taste.
ORCA 2: You're right. Let's go.