I had a huuuuuge wall-of-text comment on exactly this topic back in the old sub. Sucks that it's gone now because it answered all of your questions (and many more questions you didn't even know you had.)

I'm going to do a really sucky summary of what I wrote last time because I simply don't have the energy for a full-blown verbal-diarrhea session right now.

The multiple personality disorder thing goes waaaay back to the movie The Three Faces of Eve (1957) about a woman who has multiple personality disorder, based on a true story. There was also a book, and copycat stories/movie, which caused a public interest in the disorder.

The person known as Sybil, and her doctor Cornelia Wilbur were apparently also inspired by this story. The movie Sybil came out in 1976, also books, and renewed public interest in the disorder.

What these two movies/books/stories did was to embed multiple personality disorder into the psyche of the American public, like a meme. Everyone was aware that such a condition existed, and that it happened to some people, sometimes, rarely. (The same can be said about amnesia, for example.)

What happened in the 80's /90's? Daytime talk shows. Oprah, Geraldo, Donahue, Sally Jessie Raphael. This was the third wave of multiple personality disorder. Attention whores who'd grown up with Eve and Sybil would go on these shows and brag about sometimes HUNDREDS of personalities. There was one lady who had all sorts of people of different ages and races and everything living inside of her and she was jumping from personality to personality on camera, being a man, being a woman, being a little girl, different accents and everything. It sort of became a pissing contest for these talk show guests to out-do each other. And there was a titillation factor of childhood sexual abuse.

What went hand-in-hand with multiple personality disorder was the repressed memories. (When one of your other personalities takes over, you sometimes don't remember what happened to you because "you" were not conscious. And of course, that's where most of the childhood abuse memories hid, and it was always an exciting revelation to pull those memories out and have a satisfying conclusion of "ohhhhhhhhhhh.... so that's why I have 8 billion personalities living in my head, this explains it all!")

Repressed memories became a whole new industry for quacks and self-help gurus. Books were written about it, daytime talkshows devoted episodes to it, Roseanne Barr came out as a victim and really took the movement over the top. As /u/slushpilot said, most of us dismissed this as "those people are crazy" but there were way too many people taking this shit seriously. Many years later Roseanne has since taken back what she's said and has accused her psychiatrist of taking advantage of her, so there's that.

That's basically the answer to your question, so you can stop reading if you want but I just want to wrap up what my original point had been in that old comment.

I had called the 90's era "multiple personality disorder 3.0" Eve Being 1.0 and Sybil being 2.0. 4.0 came with the internet. In the late 90's early 2000's MPD was re-named Dissociative Identity Disorder, but on the internet it had a different name: Headmates, fictives, fictionkin, tulpas. These were anime fans who thought they had anime characters living in their heads. They INSISTED that these were people from an alternate plane of existence that just so happened to be living in their heads, despite being the intellectual property of strangers in Japan, also somehow only able to speak English and not Japanese. You couldn't argue with these people. Along with these people there were the furries and otherkin and the "fuck you, I'm a dragon!" people.

MPD 5.0 = right now. The trans-trenders. The stupid-pronoun collectors. The "I'm a gender that cannot be explained by human words" people. It's no longer about multiple personalities, but the essence is still the same - escapist fantasy that depends on other people believing your bullshit and giving you attention.

Anyway, my old write-up was so much better. This sucks. Maybe I'll re-write it someday and make a post out of it.