There has long been a disconnect, though, between the establishment of the party and the talk radio crowd. Rush used to talk about it a lot, and while people like Hannity and Levin know to stay on the establishment's good side, they'll poke at them now and then too.
The establishment Republicans know that you can't run this country with simplistic, mindless purity, but the talk radio crowd doesn't. Talk radio is binary, so they are too. So not only these people filling with rage over the Left, they were also filling with rage over the establishment of the GOP.
Trump saw this, played the game with all the right terminology (his only core belief is himself, not a political ideology), and the talk radio crowd went fuckin' nuts. The fact that he behaves like a petulant child only made them love him more, I guess because they can relate.