Sorry for being so terse, but this being a depressing subject I might as well withhold the anecdotes.

Suffice to say, many profs end up in instruction and not research by the luck of the draw, maybe helped along by a twisted sense of justice (ie, politics) from their superiors. Such discontents have contempt not only for their students, but also the job they are effectively forced to do. This of course starts a vicious cycle.

Dislike for a for teaching job causes frustrated students causes a frustrated teacher causes more dislike for the class. Hostility to the students gets reciprocated fast, both ways. In an academic environment where barring from research is the worst the administration can do, the prof feels he's lost, and there's nothing more to lose. Bad habits develop. One bad apple in a required class can discourage the majority of students in a program.