"Their conclusion was when it got too cold to raise their livestock, the Vikings just got on their boats and sailed back home." I've read that it got so cold that everything froze and they couldn't 'sail' ships...and, that many died there in Greenland, one at a time.
Their reason for rejecting that theory was that they did not find bodies in places where they normally would find them after a famine or similar event, i.e. some randomness. Also, they did not find precious possessions, just some pretty ordinary stuff. I don't remember what else, but they felt the evidence pointed to an orderly evacuation.
I don't know if we'll ever know for sure, but it's an interesting theory.