Trump is the Mule, as in Asimov's Foundation and Empire wrecker.
Sooner or later, he, or someone like him, was bound to come along, and it could have been in either party. He's just a random mutation of the type that tends to pop up every now and then, at random.
But his flock of followers is no random accident. They have been cultivated and nurtured, carefully assembled and systematically whipped into a frenzy for the last half-century or so. It's just that they've finally slipped the leash and gone out of control.