The women who identify as nonbinary (and not the trendy ones) usually start with some sort of dysphoria (compared to the men who just want to wear makeup and harass lesbians, or use it as a midpoint before going full Mtf). It's almost always about their breasts, sometimes about being uncomfortable about gender roles.
Then, after you decide you're trans, you kinda have to make a decision - am I a trans man, or am I nonbinary? The big distinction is that almost always, there's no bottom dysphoria for the nonbinary ones, while there is for the FtMs. That's basically the difference.
But then, because you're so focused on your dysphoria (being trans is a 24/7 deal), even if you hide your breasts, you still don't feel as "correct" as you were told you should, and people still recognize you as a woman (which then means they know that you have women parts, and that's embarrassing and gross). And then you start thinking about all the other ways you're a woman. Maybe you start getting super self conscious about having a soprano voice that everyone will immediately recognize as a woman's voice, and then they'll KNOW that you have breasts and are a woman. Or maybe you want to grow a beard because then, nobody will ever suspect that you have woman parts. So then, testosterone time.
Really, it's mostly just about feeling ashamed of being a woman. Internalized misogyny, sexualization, or mental illness, or whatever, that's what it all boils down to. If people started saying that it's okay to be a woman and hate your boobs, then probably half of the nonbinaries would ditch.
(this is discounting the weird ones who do hrt so they can have a beard AND dress like a stripper. That's basically just a dangerous "fuck you mom" nose ring.)
Also, women detransitioners usually blame others because they've completely ruined their lives and everyone knows it. Men can go off estrogen and everything's fine, maybe some moobs that they can hide easily, but women will need to do substantial hair removal for a long time afterwards, plus their voices are shot. I usually see women lamenting their singing voices. It's a really big pill to swallow, knowing that everybody knows how much you fucked up. Plus, the trans community is a cult that will bribe you in, so in some ways they're not really wrong: they were, in some sense, manipulated into doing it. Of course, it was ultimately their decision, but when you're young and everyone you know is telling you that you'll kill yourself without testosterone, you start to believe them. Also, women detransitioners tend to get death threats from the trans community, so it's easy to see where malice towards them comes from.
And, it's very hard to get therapy to try to overcome dysphoria. Most therapists just send you straight back to a gender clinic, and I think it might even be illegal in some states for a therapist to try to help. So the issues persist.
Like I said, this may not have made any sense, but I'm also trying to explain a mental illness. I consider dysphoria a version of OCD in some ways - how would you logically explain, "I have to touch this doorknob five times or my family will die"?
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