My understanding is that most of the women who are attacking trans women are radical feminists, as in TERF. For them, the essentialism inherent in the very existence of trans people is scary. If you believe that the only differences between men and women are anatomical, you must deny that someone can sense that they are male or female despite a body that says otherwise. If there are trans people, gender is an accommodation and not an imposition. It's easy to see how people who believe the opposite must reason backwards to the conclusion trans women are are delusional men and should be treated accordingly.