I believe the positions naturally draws people who are bullies. Not that all of them are, but the power over, and respect from the general population is probably very attractive to people who get a kick out of being asshole. Also, while I can't confirm the prevalence, there's a ton of anecdotes you here about shady hiring practices. There's the now popular story about a department refusing to hire people who had too high an IQ, and a friend told me how a department in a large city was attempting to recruit his cousin, despite him being diagnosed bipolar.