I think since dressing "ty" isn't something one can do so often without having their image go down, some take the opportunity of Halloween to do it. It gives them an excuse to try something new, or that they don't do often. In a trans way of seeing it, it could be that the person sees "sexy" as "especially feminine", and jump on the occasion of having an excuse for it, especially if they're not full-time yet.
And well, for people who'd want to try something wild for once and wear something really audacious, wearing a mask or something else that makes them hard to recognise is a pretty good way of doing it more anonymously.
Alternatively, many people see parties as ways of finding a partner, mostly, I think, for a pretty short, physical relationship. Halloween being a larger party than usual, it's only natural that such people would use the occasion to be as, uhm, sexually inviting as possible. And since a lot of people who aren't actually looking for a one-night stand do dress relatively sexy at Halloween, well, bets are up. So the non-ty become kind of ty, and the kind of ty have to go super ty to keep their status as pervert magnets.
