This is not its own illness.
Mental illness follows fashion, or at least it seems that its overt manifestations do. Crazy people are influenced by the society around them, picking up on what is current, and their delusions are formed accordingly. I believe this is why the onset of transgenderism, very little seen throughout history, was so rapid. It is more a memetic illness than a mental illness, afflicting suggestible people.
In other words, you can blame the media and the internet for the hordes of people cutting off their own body parts.
Like anorexia. A few cases got publicity, and then suddenly it was everywhere all through the 80s and 90s. Now it seems to have faded away, replaced by the new "it" problem.