It doesn't necessarily have to be altruism to make sense. Less for orcas to eat --> fewer orcas --> fewer attacks on humpback calves.

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!

Maybe the whales respond to protect weaker "cute" animals in the way humans do. When we see an injured bird, we instinctively want to help it. Maybe it's a side effect of being genetically "programmed" to protect the young of our/their own species? Maybe the "pattern matching" is matching a bit more than just the same species.

Many mammals are supposed to be able to recognize the your in other mammal species based on some fairly universal markers (large head to body ratio, larger facial features), and respond differently to younger and older members of a species. Many Dogs are known to have exhibited this behavior, and treat human babies distinctly different (more gently). I think you are correct and it's entirely possible is child-rearing instincts firing in response to other species that resemble their own young closely (other young whales) or superficially (other, smaller marine mammals).