Imagine the island as a flat piece of timber. It comes up against an immovable object as it is pushed by the currents in the sea. As the currents push, so the island tips up, the end against the immovable object remaining still and the other end tilted into the sky. It has been tilted on end.
I don't know what this passage comes from, but the implication is that the islands are made of wood, and that this wood cannot absorb the stresses put on it by the current pushing the island.