Establishment Republicans are upset because Trump knows the rules and can just about do as he pleases which is not the way the politicians play their games. Establishment politicians,
Democrats and Republicans alike have set the rules to favor their party while keeping up the charade that anyone can run. Trump upset their sandbox because he has far more than enough money to win without them and the great thing is the people know it.
Trump is running against the other Republican candidates. He’s also running against the Republican Establishment which is not as organized as the Democrats. The Republican Party has been brought under the wings of the Tea Party people aka. the Conservatives but the establishment doesn’t want the Tea Party people inside the tent. The Democrats have split into four wings; the Liberals; the Progressives, the Left, the hard Left aka. the Socialists.
There’s the Hawks and the Doves and all manner of lesser “parties” within the two major partied and Trump will brush up against all of them before he’s nominated and campaigning for President. Most of the politicians are gunning for Trump and when he’s nominated more will join against him. He is literally running from them all. And he’ll win.