I have no idea why people do things like this, but it's as old as the Internet is.
When I was on Classmates quite a bit in the late 1990s/early 2000s, I watched a woman get kicked off the site for abusive behavior three times in the space of a few months, and all three times she joined again - to the tune of $30 a year each time - under a slightly different name. She'd then post all over the groups, dropping hints she was the same person. She wanted to get caught. It was so freaking bizarre.
Once when I was on the Weight Watchers boards online, there was a woman who was so abusive and nasty to everyone that everyone knew who she was, even if they didn't post on the boards she usually did. She was that notorious. She'd join under fake personas - and again this costs MONEY - and "talk" to herself under the false personalities, posing as a troll who only targeted her each time. Every time she was called out on this she'd deny it, but then that particular troll would suddenly disappear, only to be replaced by a new one a few days later. They never targeted anyone but her, and the conversations were too personal, plus the writing style was so similar, that it was obvious she was posing as these other people.
More recently, another author and I were the victims of a romance author who takes making fake personas on Facebook and Goodreads to a higher level. He made them faster than we could out them, joining Facebook groups where one or both of us were admins, stirring up trouble, and then reporting our pen names as "fake." Gotta love the irony of that
He also would one-star all our books on Goodreads under each of the fake names. I finally emailed Goodreads, sending them links to the FOUR DOZEN PLUS names I had uncovered, advising them that there were probably more, letting them know about the same names on Facebook and what he was doing with them, and asked them to get their IT people on it and see if the names all originated from the same IP address. What a shock (NOT!) that they did, and he was banned under all
his names, including his author profile. He tried for almost a year to keep making new profiles, and every time I'd find them and send the links to Goodreads they'd delete the profile.
In the middle of all that nonsense, he sent me an email from a fake account that I didn't open because my preview pane showed me the beginning of the message: "Having fun yet blocking all those accounts from your Goodreads profile?" Right. Dude thought if he'd sent it from a fake account I wouldn't know it was him? LOL!!
He also sent an email to the other author, disclosing her address and threatening to burn down her house. She's online under her real name as well, and is very open about who she is in real life, but I'm not. She filed a police report, but he lives closer to me in real life than to her, so she wasn't really worried about him following through. She merely wanted a paper trail in case he did more stuff to her.
So yeah, this happens A LOT. It's freaking crazy. I don't understand it and I never will. You can be as careful as possible online and you still might be talking to a nut job and never know it.