That weird HH shaped head has evolved like that for a reason. Not to eat humans for certain. I think they are curious and like any animal are looking for a quick free meal.
I think they are drawn to the outflow of the bait tank... that might explain the bumping from the rear. (in my encounters no fish hanging over side but dead bait present on deck)
I don't know how to shake a shark once they are on your tail. The poking with the paddle is a good suggestion. They don't want to get hurt or be harrassed either.
The key is to stay calm. I agree best to have a buddy (hopefully a slower paddler than you). If not, PFD and VHF radio are my must haves.
So far I have not read a post about a HH attacking a kayak.
GWS on the other hand....
those will freak you out.