A variety of things, from sexual fetish, to obsession, to stress related behavior, to play, to I'm sure a whole bunch of other things that might cause a man to find relief or enjoyment in expressing typical cross-sex traits. What discomfort they may have with being a crossdresser is sociological, and most often rooted in stigma and embarrassment.
What's not driving them is a deep-rooted disconnect between their biological sex and their core gender identity, causing them clinically significant distress, and necessitating transition.
