Or perhaps it makes sense to practice the skill as much as they can, so they are better at it when it comes to defend their own young?

To me, this is what it is. If young Humpbacks are vulnerable for a certain amount of time, being good at protecting them would a good idea. They can't risk losing that skill since every time they fail, a young Humpback dies.

If the possess that level of intelligence, we will probably someday find a way to ask them.

That would be quite the impressive bit of generalized cognition and logic from a whale, perhaps even more significant than feeling something.

It wouldn't need to involve cognition and logic, it could be evolutionary coincidence: humpback whales with behavior that happens to result in fewer orcas nearby would have more surviving offspring, so this behavior would be selected for.

Right, right, I missed that completely, damn!