Minecraft science: Zombie Pigmen accumulating momentum in lava? - confirmed.

EDIT: Patched in 1.9.5
I mentioned I'd read this ages ago to Roboticaust during his livestream and he said he hadn't heard of that before so here's a quick video showing it in action.

Zombie pigmen and blazes are the two nether mobs that will always attempt to swim up in lava. Only difference being blazes can fly out if they have the clearance above. In the case of a lava column both mobs will climb until they hit an obstruction and get stuck or exit the lava. After a few blocks in height they will make no effort to escape the column.