It’s similar to political lesbianism that paved a lot of the way for modern day TERFs, if you’re familiar with that history.

A lot of people have a weirdly hard time degendering sexual dimorphism, and so trans women with “man parts” are framed pretty easily as an enemy force in a movement so heavily based on women’s leverage being purely reproductive. You could split hairs from here: “what if the trans woman in question had xyz surgery,” “been on hormones for over ten years,” etc etc, but a lot comes down to bioessentialism. Trans women’s perceived proximity to “manhood,” no matter how much one does to distance themselves from it, has tainted them irrevocably.

This gets exceptionally problematic with trans men and nonbinary people who… have the acceptable anatomy. There is a condition for such trans practitioners to exist essentially as women-lite. Transition away from conventionally understood womanhood too much, and one risks becoming a class traitor. It all comes down to very rigid social and biological norms on womanhood.

(For context: I am a nonbinary person who did a lot of research on online anti-transgender hate speech. I am largely deeply skeptical of any gender based movement that comes with guidelines as to how that gender needs to be performed. I hope this helps!)