The basic chain of events is:

Teachers wanted to have books like this: https://www.youtube.com/watch?v=_Cd5VUhMrF4 available in classrooms (this is on the MO 'banned books' list). Parents (obviously) objected to this, but the teachers/librarians dug their heels in.

Due to a loss of trust, parents started demanding oversight into everything that the schools/libraries were making available, and that's how you end up with absolutely crazy things like banning photos of the statue of David.