I don't know the percentage, although it is small, but the reasons are varied. The main reason, supposedly, is sometimes, at least from F-M transitions, the person was lesbian instead of trans. In the case of, say, Chaz Bono, as Chastity, she thought she was lesbian, but realized years later that she was trans. No regret there. Sometimes with as much hatred of male homosexuality, the thought of being a hetero trans woman is more "palatable" than being a gay cis male.
IMO, there has become a very narrow window for how we express gender. It was always pretty narrow for guys, unfortunately, but now it's narrowed for girls. Not every girl wants frilly, pink, unicorn and mermaid clothes, so what is a girl to do when it's expected for a girl to be super girly but they are not? I wasn't super girly when I was a kid. Of course, clothes came in many colors then, pink being a small number. Why name a not so girly girl a "tom boy?" Where did they even get that term? If I were a kid now, perhaps I would question myself because I didn't like to wear those types of clothes. People should just be allowed to be who they are, whether they are girly or not so girly girls, or macho or not so macho guys. It's already hard enough to be a kid without going overboard with colors, styles, mannerisms, etc.