Playing? Big guy helping little guy? Bully? Your call...

I've seen video of dolphins killing porpoises, so I wouldn't be surprised if this were an attack by the whale.

Nah, don't think so. A whale moves about one-tenth the speed of the dolphin. To hit it this way the dolphin had to be dead, asleep or seriously injured. I think it's more of a playful nudge.

I know dolphins love to leap ahead of a fast moving ship (called bow-riding). Maybe the dolphin was wanting the whale to move so it could play in it's wake ... or, maybe they were having a heated discussion.

‘It is the behavior of animals that tells us about their mental capacities, not brain size or anatomy. Anyone who has interacted with dolphins usually swiftly and intuitively realizes that they are meeting intelligent, emotional and extraordinary animals’
http://www.wdcs.org/dan/publishing.nsf/allnews/713EDD5FDA2E042B802571FB0057B0DF