Or hell, who says that Leviathan's back needs to be in the same place his front is?
Have you seen how long he is? Could well be head and tail are different targets and the tail always has the back facing towards the party from a different direction. It'd have some eventual tail swipe attacks while the head does most of the heavy damage.

He moves them around the ship, the tank tries to keep the head on him so he doesn't obliterates the party while the DPS focus on the tail.