I don't claim to be able to answer this question definitively, but I was spectating on 18 while Watson was playing.
When Watson's group was approaching their drives they were clearly upset about the pace of play, which was dismal- approaching six hours. Mind you the weather was brutal, but all the players in the group played very quickly as soon as they had the chance.
I don't believe he "tanked," but I do believe all the players in the group had completely lost concentration and were just trying to get the hell off the course. He did not "tank," but I'm not sure that he really cared at that point. Did you see his body language on those bunker shots? It seemed to say, "who needs this?" He probably assumed he had missed the cut anyway.