In identical twins the rate at which both are trans if one is trans is statistically significantly higher than the rate in fraternal twins. That suggests that there is at least some genetic component to being transgender. However since the rate isn't 100% (if one identical twin is trans the other sometimes isn't) its not 100% genetic.

Essentially there's some genetics to it, and probably some environmental aspect as well. Though that doesn't mean "well you had trauma so that's why you're trans" or shit like that, it means like possibly hormones in the womb, or possibly other things, we just don't really know, because its not studied that much.