This is because the physics FiNCK uses are far from perfect (not Nifflas' fault, though he could've used a newer version of the physics extension instead of the old one). When you're holding something and land, there's a slight possibility that the thing you're holding can 'push' you a bit left&right. You can see the same if you pile tiles and jump on the pile - some of the lower ones may move a bit. So in this case the physics decided to bump you a bit to either direction, thus killing you.