I could not even begin to speculate; it would be from my hindquarters.
Believe it or not, I've actually only had very few experiences with "bad" GMs at all. One of them was quite bad, but most of the rest were just really disappointing. But I've had many experiences with very mediocre GMs, about as many as I've had with very very good GMs.
I just really, really don't like having my behavior being limited by rules I cannot understand, am not allowed to know or see, and cannot tell when they've been modified or broken or "adjusted" or whatever, all while the other person (or people, but usually only one person) has their behavior mostly excused by such rules....particularly when they are the only one allowed to decide when and how these unwritten, unspoken, invisible, inaccessible rules change.
I've dealt with that from a couple former friends (who I am very glad to not have in my life anymore), I had to slowly but surely push back against that from a relative (the irony being that they always hated it when another mutual relative had done that to them), and have had to navigate exhausting and infuriating labyrinths of that sort of thing just to begin integrating into a wider professional setting more than once. I never, ever want to have to deal with that BS in my gaming. Ever.
When we speak openly and honestly with one another, set rules and abide by them unless we intentionally set them aside to talk the solution out like adults, and treat each other with respect and support, there is no need for these invisible bindings that only one side gets to modify whenever and wherever and however they like.