It real simple.
GOP voters hate the DC Establishment. They have been continually betrayed by their Party Leaders. So they are looking for someone to go clean out the DC stables. These same people are not comfortable with Trumps bombast and loose cannon behavior. Carson is anti Establishment without all of Trumps baggage.
Trump gets away from this loose cannon ways and hammers his economic message harder and harder, and he can win over those Carson voters to him.
What he cannot do is talk about Bush and 09-11 like a Democrat and expect the same people who elected Bush 2xs to look with favor on his candidacy
Yup.
I think this dalliance with quasi 9-11 truther nonsense is what helped Carson cut into Trump's lead. Trump should have never gone down that road, he almost sounded like Michael Moore or some InfoWars paulbot lunatic.