Not aware of his history and didn't hear his rant, but he was one of a few primary candidates over whom I preferred Trump. He was the MSM's chosen, like that Governor of UT in 2012 and McCain in 2008. The MSM choose tame RINOs they can set up in the primary and then knock down during the campaign for November. So Kasich was a, "Oh please, dear God, NO!" candidate in my view.
Not having heard his rant, my guesses would be that he has either taken his dumping poorly, or he's showing his true colors. Or both.