I think he was annoyed/bored. He is basically playing against a computer for an increasing number of moves. By the time the theory ends, the game is near-decided. Chess960 reveals the natural talents over the memorizers.

The chess lovers complain and complain, because everybody's style is starting to mimick closer to the objective best moves.

It is also a psychological move in trolling Caruana (part of the game): "You did not even try to win with white in your last game before the rapids. I'll draw in a superior position because I love my chances there."