Humping is a way the dog shows dominance. Maybe Hank felt threatened so he wanted to show that he was superior to Mike. (I have no idea, just a guess)
Zoee and Taggart just started the humpty dance (as we call it) today. Zoee did it earlier to him and then he did it tonight to her.

Perhaps someone else will be able to give a more accurate reasoning to this. But as I've been told it is perfectly normal. We discourage it around here though.