I believe that the original Trump plan was to announce his run at his latest rally but he changed his mind while travelling to the venue. I think that because he brought the family with him which is unusual.

He also said that he thought about making an announcement but he thought he would leave it until after the election to avoid the distraction.

I feel now that he would have been better adding the line then and not generating a massive build-up to last night at Mar a Lago.