There was an article from either his 2008 or his 2012 presidential run where some reporter was embedded into his campaign. It was absolutely hilarious, wish I could track down a link.
Anyway, apparently it was completely and transparently a bid for attention, mostly to the benefit of Trump's resorts and hotels. Like, he knew there was no chance at all of being taken seriously and he was just saying outlandish things so the press would cover him. It was essentially free advertising for the Trump "brand." And everyone knew it and the reporter just sort of gradually pieced it together over the time he was there.
I think this campaign started out the same, Donald probably assumed he would be a non-factor by June of last year so he entered and started saying even more outlandish things than ever before, as he needed to in order to get attention. And lo and behold, it turns out that about a quarter of the Republican party are straight-up fascists, which everyone pretty much knew already but no one wanted to talk about, ESPECIALLY the non-fascist Republicans.
So this time, when he went really left field to try to promote his brand, and started saying things about all Mexicans being rapists and mocking a handicapped reporter and the like, those people rallied around him for the first time. And I think what's happened now is that he's gradually started to believe his own shtick, like "Yeah, the people really DO want me to 'make America great' so I should be president for real!" Like, before it was just a publicity stunt and a fantasy, but he really thinks he'd make a good president and so somewhere around August or so he started taking it seriously.