If the whole earth were covered in what is called today continental crust (less dense rock, constituting the continents and their shelves), but then some cataclysm caused most of it, perhaps from on one side of the planet in particular, then the remaining crust might have separated somewhere near it's middle from tension (rock is strong against compressive force but very weak in tension, twisting, bending, and shearing forces), and then drifted on its underlying hydroplate until the hydroplate all came out, at which point the continents would just grind to a halt wherever they happened to be, creating a lot of lava or magma in the halting process (all the momentum from the drifting continents has to go somewhere afterall, if they're just going to up and stop), earthquake activity, some of which might still be playing itself out even today, somewhere, maybe.