To be perfectly blunt and without the typical humor that's been associated with him... I would say it could be a fear he sees of him losing ground with the realization that all of his problems and misdeeds are finally catching up to him and one of the very few paths out is slipping away.
For someone who could be seemingly suffering from what may be, or something similar to NPD; seeing a lot of what one created, appearing at least to their eyes, to be starting to erode can be very distressing to one's psyche. This I imagine would cause that person to lash out, similar to how one would act in a fight or flight scenario.
I have a question. Why is it when a politician says positive things like their going to "lower the taxes" and "improve quality if life", etc. when elected everybody knows they're talking out of their ass and none of that is going to happen. But if a politician spews negative shit everybody acts like it's all already set in stone and almost guaranteed to happen. In my experience politics is very slow, big changes happen slow and often don't stick past a legislative period. Most politicians are all talk no bite, this has been proven again and again and again.
Potential damage. It's much easier to destroy things than to build them, and the damage can be done quickly. (Even more so when a handpicked court gives the president more power.) People should fear deranged fascists and keep them out of office.