You could argue that the cause is being bullied as a child, but you can also argue that the cause is autism. I'd say both are true, but as long as the solution is the same it doesn't really matter.
Unfortunately it's a bit more complicated than that in reality. Many high-functioning autistic people are often initially misdiagnosed with bipolar, borderline, ocd, add, narcissism, schizoid, etc, for example, because their symptoms appear the same. But the underlying causes, or internal mechanism can be quite different, and the same goes for solutions. On top of that, it's quite common for autistic people to have both autism and, say, ADD.
Personally I feel that we often spend too much time thinking about causes, perhaps because of the history of psychology. Causes are only relevant insofar that they require different solutions, and spending too much time dwelling on a cause can often itself become a problem.
What does this mean?
The singular "they" is already commonly used when the gender of a person is not known, so its extension into non-binary genders is quite sensible, in my opinion. Some others who regard themselves as a non-binary gender prefer invented pronouns such as "xe" or "ze", but I think expecting others to remember and use these is rather unrealistic, whereas "they" has the advantage of already having mainstream use in similar contexts.
In English one refers to a male as "him" and a female as "her". However, some people identify as "non-binary", which means that in their own internal self they so not feel that they are entirely male, or entirely female. As a result, they find it not quite right to be referred to as "him", but also not quite right to be referred to as "her".
So they prefer to be referred to in the gender-neutral "them".
If you are "traditionally aligned", if your personal self-identity matches the body your ghost[0] inhabits, this can be totally mystifying. Personally, I've come to accept that people are weird, and if someone says they want to be referred to as "they" and "them" then why not?
[0] "You are a ghost driving a meat covered skeleton made from stardust[1] riding a rock floating through space."
[1] Or nuclear waste - take your pick.
Edit: To the people downvoting this, it was intended to be a factual answer to a direct and reasonable question. I have no idea why you think this comment is inappropriate and deserves a downvote.
Is this common? Never heard of it.
I don't know how common it is, but you can look up the terms "non-binary" and "transgender" and find out more.
Again, personally, I think we should just let people get on with being who they are. Respect them for who they are and the work they do, and just stay the hell out of their personal lives. No business of mine.
Yes, it's rare, but not as rare as graduating from Harvard, and it's definitely not just an Internet phenomenon. There are just a lot of areas where they don't feel welcome. They're real people in real life and they tend to congregate in places where they'll be treated as such. Right now that includes sections of the Internet, but also quite a few real life enclaves.
I like doing this because it's one small, simple way of welcoming those who might not otherwise feel accepted.
I don't think that there are really solid numbers for how different people identify themselves. The information would have to be self-reported, and since there would still be some social stigma attached in many places, I doubt that a survey would end up being very accurate.
[0]: https://en.wikipedia.org/wiki/Gender_dysphoria
Edit: It seems it maybe caused by biological factors as well