Why is it always PE teachers? Same rumour was going around my school.
"Those who can, do; those who can't do, teach. Those who can't teach, teach gym."
Basically, the stereotype is that gym teachers are always some flavor of mildly incompetent former high school jock who wants people to respect them while putting in the minimal amount of effort; combine that with them
also stereotypically being former bullies and the power being a teacher gives one and, well, it's not hard to figure out where their minds would go.
Obviously, not all gym teachers are like that, but as they say, where there's smoke, there's fire.