I was pretty far ahead, but Lord Rat bought the last Province and ended the game with me in the lead. He could have hoped to buy a couple of Duchies to close the gap before I got to the last Province. Then it would have been a race to that last card.
Even more mystifying, he used a Chapel to trash an Estate on that last turn, the one where he ended the game.
#132612 (I think... that's at the bottom of the list)