However, it is trickier to explain apparent altruism directed towards other species. Pitman and his colleagues explain that for the humpback whale, this intervention on behalf of other species is a “spillover” behaviour. They suggest it is an extension of the humpback whales’ “drive” to protect their own calves.
Humpbacks may have learned to respond to vocalisations of attacking killer whales, which trigger them to drive the killer whales away, regardless of the species being attacked.
If this tendency to drive away killer whales whenever they are attacking has helped humpbacks to protect their own calves, then the genes that promote it could be maintained in the population, even if other species benefit at times. This interspecies altruistic behaviour may be “inadvertent” altruism – it can be altruism in the individual case but it is ultimately driven by self-interest.
- More Here but in other words "We don't have a damn clue!!"