This is a good 60% or more of all male trans people, to the point they're called Autogynophiles, and likely a big componenet to the recent Yaoi to Pooner pipeline for female trans people. Females seem like they have a more diverse range of reasons, including desexualization (AHEM AUTUMN).
An inability to cope with adulthood, disconnect from the body and brain due to a disorder like autism where the brain has trouble connecting, fitting in with a peer group or rebelling against another, wanting special treatment, gaining friends and acclaim instantly by declaring you're trans when you didn't have much of either before, body modification love, an excuse to get surgery for anorexic and other body dysmorphic types, are some but not all of the possible reasons people troon out.
Considering how much furry fanservice and porn Autumn churns out, it would be more astounding to find someone who transitioned for non sexual reasons than sexual ones.

You probably know this, thus the tone of your post, but I have to point it out for anyone unfamiliar.