I believe that Dr. Carson just didn’t hear the signal that he was supposed to proceed to the stage. I noticed how the others directly after him, not sure who they were, just breezed on by him without a thought. It was Mr. Trump who clued in that he was standing there when he shouldn’t have been, and perhaps thought that something was wrong.
I though it was very telling that he stopped and stayed with Dr. Carson and made sure there wasn’t a problem, then they both went out individually after they were called upon. It showed character, IMHO.
Thanks for the explanation!