Difficult/opaque lecture style, very high difficulty tests, openly mocking students in a 500+ person lecture if he deemed a question not intelligent enough, and on top of all of it, forced to buy a textbook he co-authored (homework questions taken from the required book, refreshed every year).
For some reason this story seems to happen more often in chemistry than other disciplines. In my experience as a Biomedical Engineering major, my chemistry classes were by far the most difficult, almost like it was the point - just difficulty for difficulty's sake. And I had plenty of other hard classes, including AI/ML, Diff EQ, and a course on the physics of blood, but each of those professors, while rigorous, were also fair and kind, willing to help students understand the material, and definitely never refusing to answer what they believed to be a "stupid question."
I even remember conversations with friends about how the chemistry department seemed to have a very high number of massively egotistical professors who mistreated their students. My chem major friends confirmed it was like this all the time.
All of this is anecdotal, but soft-confirmed by multiple friends and friends of friends at the time. When this story broke I was honestly not at all surprised.