i'm not sure why the documentary suggests that the only options are protecting the calf or predatory behaviour. rather, the more obvious explanation is competition over resources. you'd imagine that the orca would be more interested in the seals, and are just trying to scatter the sharks out. if they can get some food out of it, great. but that's not really the point.

it's just more evidence that orcas are evolving to occupy the niche that sharks currently occupy. nor is this shocking - the era of mammal diversification and replacement is the era we're continuing to live through. bye sharks...

http://www.youtube.com/watch?v=Ay4xnI216iY