What would cause a mega-continent to hold together for so long and then suddenly (relatively speaking) drift thousands of miles apart?
It happens a lot - every half-billion years or so. Rodinia, Pangaea, Godwanaland.... Remember, our continental plates are just the traces of slag left over from when this place was a molten ball. They are incidental to the big convective patterns that drive the "continental drift" that we see on the surface.