I think the answer is pretty simple. That is exactly what pressure, and expectation, can do to a person. Mr McCain was conscious of every word that he uttered, of every speech, and every phrase, that he used, always worrying, always wary, that his words might be used against him. And in that muddle, Puranjoy, was lost the old McCain.
Only, things like this should not happen to the President of a Nation. Is that why Mr Obama was the worthy choice?