I noticed the change a few months ago after Trump tweeted “Rush is Fantastic”. Rush at that time was talking about how briliiantly Trump was dominating the drive-bys. It was practically the next day that Rush’s whole tone and conversation started going hostile toward Trump. I also have noticed that Trump used to always talk in his rallys about how great Rush was. Ever since that tweet, he NEVER mentions him. I think somebody got to Rush and like you said, and applied the vice grip.
Rush changed after Rubio mentioned Rush was at the Gang of 8 amnesty push GOPe dinner. Rush was exposed for what he is. Prior to that Rush gushed about Rubio. “Rubio is Reaganesque.” Then all media dropped Rubio like a hot potato and the rest is history.
And then the attacks on Trump and building up Cruz. Rush was exposed as GOPe. The opposite of the character he plays on the radio.