There was one play where he was scrambling to his left and threw the ball away before getting knocked down pretty hard. I guess that could have been where it happened. It also could have been something he re-aggravated in practice leading up to the game, and with the way our protection looked in the 1st half, the coaches may have decided to get him out of there.
He has worn a brace all season following the SC game where he was injured, but the brace was off as he walked off the field at halftime. He had a noticeable limp, so I don't think it's a coverup situation for something. He made big effort to get from Honolulu to Seattle to Arizona to El Paso for the game, so it would be strange to assume he suddenly decided to go off in the locker room at halftime. That would have leaked by now I think.