Alright, I subscribed to comment. I am transfem, currently taking estrogen, and my arousal change generally lines up with average change for people on estrogen. Perhaps a lot of people have already written about their experiences around their sexual preference changes elsewhere, but there's 0 comments here now so I might as write here.
I guess there are two factors here. (gender factor) Accepting you are a different gender than you thought before transitioning. (trans factor) Accepting that you are trans, that being trans is okay.
These are complicated, but I'm gonna try unwinding them here.
Regarding gender driven changes, in my own experience they were at least 50% social. For me, these were an increase in arousal around being generally submissive, stemming from feeling and interacting socially as a woman, and increased in arousal in men, stemming from hormones and from the stigma against gayness (it feels less socially acceptable to like a woman as a woman).
I feel like part of these is a social effect, that I even started to feel before I was on hormones but after I started to accept my gender. The hormones did accentuate the effect though. I've heard a couple of reports from other trans women that had similar experiences. It would be really interesting to hear reports from trans men as well.
Another effect that I underestimated before I started transitioning is the effect of accepting that I am trans myself. Honestly, I accepted I was trans before I fully accepted I was a woman. It's a strange feeling to realize you're trans, you have to accept that your thoughts are different than most people. It's that much more special and relieving to find a group of people with thoughts that actually match your own. I feel like trans people really see my full me and get my story, and I find that connection hot. I guess this factor drives people to seek T4T relationships.
So I feel like it's not about being exactly part of the trans community, but a sense of shared experience. That's why even people who haven't realized they're trans (eggs) are often drawn to trans people.
The increased arousal for mythical creatures or exhibitionism/voyeurism I think is also tied to how trans people are generally perceived within society, as some different creatures and much more sexualized. Sometimes, sex can be a release from anxiety of public perception, by releasing the tension from past interaction dynamics. It can feel good to play out that dynamic but somehow feel in control.