I should probably chime in here seeing as it was my memories topic which spawned this one.
The only time that I was abused by anything other than myself was when I was thirteen and was sexually assaulted by my best friend. At that point I was still thinking that I was a gay female, however there had been signs from before that I was trans, even though I hadn't put the pieces together. For example, one of the only things I remember from being five or six was being on a school trip and overhearing my friend tell the teacher about her cousin who was transitioning from female to male, and for some reason hearing about that made me feel really awkward. Then there was my thoughts that it would be awesome if we could choose which gender we wanted to be in the womb by pulling a lever made out of broccoli (yes, I was incredibly young then and thought that babies grew in the stomach), and that if I'd had the chance to choose I would have been a boy, but never have a 'sex change' because I thought only freaks did that. And probably the biggest warning sign that I was trans would be when my breasts started to develop and I would stand in front of the mirror every weekend stretching my arms out to try and convince myself that I wasn't going through puberty because it was just so wrong.
So although I don't have the "typical" trans narrative of always knowing (is that really the typical thing these days?), there have always been signs for me, so I don't think it was triggered by the only real case of abuse in my life. The depression and other things caused me to abuse myself, but I've never known the root cause of that, maybe it was being trans, maybe it was being put second to my parents work all the time, or maybe I just chemically lean that way, I don't know. No matter what it is the signs have always been there now that I know what to look for.
I think that there could be some people, particularly females, who decide to transition and believe that they are trans because of things which have happened in their past, but I think that those people are not trans in the same way that most of us are. They are probably -- and this is all speculation -- more bothered by the social aspects of being their assigned sex, rather than the physical, and might not even feel dysphoria, just hatred of the parts which drew their abuser to them (if it was sexual), and this gets confused. So I think that for the people who were trans before abuse transitioning is a case of freedom from their incorrect body, but for those who were trans because they were abused it's more of escaping from the situation that lead to them getting abused. Of course I don't mean to offend anyone with this paragraph, and please correct me if my guesses are wrong, this just makes a bit of sense to me. People are free to transition for whatever reasons they like, and I'm not saying that anyone is "more trans" than others; you can life as a martian for all I care, as long as you don't harm others.
As for lots of trans people seeming to have histories of abuse, I think that some of that might be to do with those people who have more difficult backgrounds and transitions coming to support places like these. We rarely hear about those who have relatively pain free transitions, plenty of money, and their whole world behind them. All too often the people who come here are looking for support because they're struggling, and those people seem to be more likely to have had a history of abuse, or maybe their struggles are caused by their abuse. This is just generalisations from what I've seen around here and other places on the internet, some of you could be billionaires for all I know. Just that people who have had these histories are going to mention them, no one is going to start a topic called "I was never abused, and my life has and will always be hunky dory." And because we're already revealing one of the most personal and painful bits of our lives, and sometimes intimate parts, people will be more open to talk about other things. I know that for me personally, this is one of the few places where I will even mention the sexual assault, in real life I don't ever talk about it, not even to my girlfriend except, "Oh I was just getting bothered thinking about when my friend f-d me over again."