Maybe he was confused, I know I was when BJ was hurt. BJ is known for his ability to take punishment, that was the first time we've ever seen him really rocked. He was obviously being battered by GSP and Diaz but he never had that look on his face like he did against Rory like a light breeze would make him go down.
My first thought was "WTF, is he playing possum?" I honestly thought he might be faking it to draw Rory in and land a big shot. In retrospect that seems silly but live, at the time it was the first thing in my head, maybe it was the first thing in Macdonald's head too.