last ice age, there was heaps of ice around the poles, and that ice has mass. Over time, gravity pulled that mass down, which caused the ice free equator to bulge up. Then the ice age ended and the ice went away, and ever since then the planet has been adjusting - the equator pulling down and the poles raising up. Like the spinning ice skater pulling her arms in, the equator pulling in speeds the planet's spin up.
But if this was the case, why hasn't this been a steady speed up over the last hundred years? Why the steady slow down, then sudden speed up? Global warming comes to mind, as it does in everything, but the mass of glacial ice lost is too small and too recent to be having this effect, surely?
Everyone's pretty confident that this anomalous speed up will end soon, and we'll return to the expected steady deceleration.