So, I've had recent, very personal experience with the whole gender thing. We are currently estranged from my daughter and her family, for reasons that are irrelevant. I still see her posts on Facebook, though. On "National Coming Out Day," my daughter came out as non-binary/genderfluid. She says she sometimes feels womanly and sometimes feels manly and sometimes neither. She says she is fine with "he/she/they," as they all fit and she doesn't want to give people a hard time when she doesn't even exactly know what she's feeling just yet. She says this doesn't really change anything about her, she isn't "trans" per say, but she doesn't feel like any one particular gender.
Now, growing up, my daughter never really expressed any confusion about her gender. However, she has always been kind of free thinking on these matters. She will often dress as male celebrities/characters for Halloween. She also dresses as "sexy" female characters (I don't mean that to sound weird, but that's what they are - "sexy" nurse, slave Leia, etc). Day to day, she usually chose female clothing but was also comfortable in t-shirt and jeans. Outside of costumes, she has never dressed or acted more "manly," whatever that actually means. So I've always known that she thinks very openly about gender matters but I've never seen any signs of some deep-seated gender conflict in her from birth to age 30, when we stopped talking. We were very close before that and we spoke openly with each other when she reached adulthood, so I think I would have had some clue . . . but maybe not?
She has always been a staunch supporter of LGBTQIA+ rights. We have had some disagreements about this because, full disclosure (and this really shouldn't be a surprise to anyone who has seen my arguments on this forum), I used to have some pretty wrongheaded ideas about these things (eh, I probably still do, but I'm trying to be a better person). In fact, arguing with her is one of the reasons I've changed my mind on a lot of these issues.
I think my daughter is an excellent example of the whole "social contagion" thing. It's cool to be queer. She has a natural affinity to those causes and many of her favorite celebs have come out (Demi Lovato, Eliot Page) and she wants to be a part of it on a more personal level. As she herself says, this changes nothing about her fundamentally -she's still a mom, still a wife, still female. She just doesn't always feel like a woman or a man. I really don't think she's saying anything different than what Emily's Cat shared about herself earlier or even what I have said about myself. What's happening with my daughter is that she is from a different generation that 1)has a much more open mind about sexuality and gender and 2) feels this narcissistic need to label themselves, whereas EC and I are from a generation where there was more taboo around sexuality and gender and we were kind of forced to conform to certain roles.
TL;DR: My daughter coming out as non-binary only reinforces, because I know her on pretty deep level, my view that much of this current shift in "personal gender identity" is rooted in narcissism and social dynamics, rather than any deeply-held internal "identity."