All you really need to know is that it's a hell of a lot easier to teach social justice than to understand the finer points of literature and walk kids through a difficult read.
The marking is a lot easier too. A teacher barely has to read an essay on BLM or feminism to assign a mark.
The real problem is that there's no nuance involved in social justice, which makes it fucking easy to teach. There's no need to engage in any critical thinking whatsoever or to teach it. It's a bunch of easily understandable points for the students to regurgitate, nothing more.
It isn't that teachers turned into activists, more that the quality of teacher has massively declined in public schools and the curriculum has changed along with it.