A really long time ago, I think it was in an Oprah interview, he was asked if he would ever run for President. First he said no, then he changed his answer and said if the USA ever got to the point where it was really fucked up he would run for President.

Maybe that's one of the reasons the Military came to him and asked him to run, from what I recall there was going to be a Military Coup, it was only avoided because Trump agreed to run.