@alyx My experience is similar, but the first thing that comes to mind is that it might be some sort of a filter bubble that you (and I), personally, are trapped in. Some complex non-intuitive filter causing you to encounter a higher concentration of trans people online for some reason. Confirmation bias can play a role here too.
But then there's the idea that the wired is the frontier of today, and all oppressed, closeted undercover, and isolated people seem to gravitate towards it because they can express themselves, often anonymously, in a safer manner, and use various online filters to increase the chances of meeting others like them. Trans people, Nazis, ex-Muslim apostates, pedophiles, Saudi women rights activists, etc. And as mainstream social media starts reflecting the stigmas and ideological wars of the real world with the floods of average people joining it constantly, the marginalized groups start getting driven away from them and into new frontiers, like the Fediverse.
Well, there might be some other coincidental, but more concrete, reasons for the fedi to be like that. Maybe, for example, some of the first instance admins was a trans person, and they got their trans friends on it, then a network effect started pulling more and more of them into the fedi, starting new instances, and joining existing ones. That admin could have been equally a gardening enthusiast, and then the fedi would have been full of gardeners and florists.
But what do I know