In 89 percent of the recorded incidents, the humpbacks seemed to intervene only as the killer whales began their hunt, or when they were already engaged in a hunt. It seems clear from the data that the humpback whales are choosing to interact with the orcas specifically to interrupt their hunts. Among the animals that have been observed being rescued by humpback whales were California sea lions, ocean sunfish, harbor seals, and gray whales.
So the question is: Why are humpback whales doing this?
Gotta love nature.