It's less so a standalone or sudden urge to harm yourself, and more of a result of prolonged distress and desperation that peaks when you're feeling especially bad. Gender dysphoria doesn't really stop much, but it can intensify, or it can hide and linger in the background. If you're under influence or sleep-deprived or very depressed or affected by other mental illness, what's on your mind isn't super rational and consequences are secondary to what you're currently experiencing. Like, 'it can't get much worse than this'. I don't know if there's psychological or neurological research into it, but from my understanding as a trans person, the brain has a sort of a expectation of what body parts it maps onto. So having that extra organ can feel viscerally alien. In my experience, before mastectomy, seeing the beasts on my chest felt really weird and uncomfortable, and, notably, things touching that tissue kinda freaked me out, like my body wasn't supposed to be there and I shouldn't feel anything from that area. Of course, people's experiences and intensity of gender dysphoria varies. But this phenomenon of a body part feeling like it's not your own might make it easier to hurt yourself, if you're feeling bad enough. I want to add that after my top surgery, my chest immediately felt normal, I'm nearly three years post-op and it's a great relief.