Apparently, Trump has convinced his CULT that were he to be impeached, that would basically be a coup (as he has often stated) of his administration.
This buffoon-in-chief is asking his base to ignore Article I of our Constitution and existing laws, and yield to him the powers of a wanna-be despot for whom laws are irrelevant.
The reason people think it's a coup is because, the Democrats left no doubt their goal was to remove trump. Even before an investigation had started, the dems had him pegged as guilty and were talking removal. Heck, I think even before his inauguration there were some already talking about methods of removing him.
You see, impeachment is necessary for high crimes, but when you broadcast how badly you want him gone, and all of the rhetoric and hate that has been pointed his direction, its starts to look more like a coup.