Having been in my fair share of trans chats on Kik, met a solid handful of trannies irl and been to two conferences where 100+ trannies convened, I can absolutely assure you that the stats are correct: most aren't just suffering from GD... there is a high rate of depression, autism, bipolar, and even multiple personality. I cannot be certain why this is, but I have a theory that a fair portion are transtrenders... it seems like "being trans" in a post-Caitlyn (she's not the root cause or being emulated imo, but it WAS a social mile marker) is the equivalent to being punk / emo / goth a decade or two ago. It has reached a pivotal sweet spot in that it is common enough to not be completely ostracized in society, yet still weird enough and socially faux-pas enough to be edgy and fringe. Also, as far as how this links to the other mental issues, it could be a number of ways sufferers could latch onto it for benefit: providing a reason for being different to begin with (autism); something to fixate on (autism); something they think will make them happy (depression); an explanation for their manifestations of personalities through the host (MP), attributing the light and dark mood swings to a more convenient binary explanation -- i.e. it's my female self being dominant when I'm positive (bipolar). I'm just tossing out the line here... I honestly am only speculating.
The high indicator and/or red flag that this is a trend, to me, is the fact that the vast majority of non-binary transgender people are young (and ugly, but that's just being mean). It's bad enough explaining to people what GD is and being a transsexual (take it from me: this IS the fundamental, primal form of transgenderism, and the form that will be provable by science), but then you have these other clowns hijacking the trans-flag and demanding their zhe/zhey pronouns be respected and twisting the public opinion of what transgenderism really is. Ugh... fucking frustrating. I could go on for hours over this bullshit, but I'll spare you all my rant soapbox and just stop here.
I will say this though: I am very very much looking forward to the future of transgenderism in the world. My big prediction? By 2030, the rate of which individuals who identify as transgender will decrease noticeably.