This is actually an interesting observation...
I have a hypothesis. Transexuals have trouble fitting in, and so have trouble gaining the social favor required to shoe into many professions. Of course being a sales associate or model or TV news anchor is out of the question, but even if you're not public-facing, you might face a lot of misunderstanding and outright mistreatment in an office or factory setting.
Computer science is one of the most meritocratic disciplines that exists. If you can code, you can code, and everyone wants your code. If you can design, config, debug, etc., then by God, plez HALp us.
So many "trannies" flock to CS because their appearance or identity is less likely to be an obstacle to their success.