I missed it on ESPN, but I saw the match on youtube. It looked like Barnes came around the ball more in the 4th frame and tripped the 4. You can see the ball had more side rotation than the previous shots. In the 6th frame light hit, it looked like he moved his feet and stayed behind the ball, like all the other frames except the 4th. In the 8 frame split, he came around the ball again and may have moved his feet back. Please check out the amount of side rotation on the ball in the 4th and 8th frames vs the other frames.
Did Barnes choke, I would say no. I would say he made an adjustment based on an off shot and he did throw a couple of off shots. Plus Fagen put up a score.