At my small highschool it was well known that the teachers essentially rotated being principal. They all hated it but it had to be done. While I was there it was the history teacher. Before that it was the science teacher. After I left the english teacher took over the role. Yes it was <100 people so there really was only one teacher for each subject with some overlap.
And if that's the only power you have in your life, you'll protect it viciously.
Teachers are usually in it for the warm fuzzy feeling of doing something good, but I've never met a headmaster who didn't behave like I described above.