If I had to guess, it's part defense mechanism, part exasperation. Defense mechanism because misogyny is a pretty insidious thing, and like racism, it's also percieved as harsh to accuse someone of it.

Exasperation, because, for me at least, you see more and more of the label being thrown around, and the response turns from "Hmm, this is a bad thing, good to know!", to "Jeez, I GET IT ALREADY. Sexism is bad! Enough!"

To use TVTropes terminology, anvils are falling from the sky like raindrops. One of my favorite webcomics, Sinfest (http://sinfest.net) has been beating this drum non stop for the past month or two.