I'd always assumed that it was because there was an actual "hump" on the back of the whale... What I learned from whalespotting though, is that they surface differently from Minkes, making more of a hump shape as they dive back down. It's a very distinctive movement, and I can see why the old whalers would have quickly named the Humpback Whales as they did. Just as the whale that's called the "Right" Whale got its name... It wasn't because of anything to do with left or right, but because that particular type of whale had a high percentage of oil in its body, that allowed it to float, even when dead. This made it one of the easiest whales for the whalers to deal with, plus one of the most lucrative, so it was the "right" whale to hunt.