I think some people just like the idea of being at the centre of a great big palaver... that they started.
A lot of people feel highly disenfranchised and invisible in their own lives. The current internet/social media/reality TV climate shows a particular kind of exceptionalism and specialness to be desirable and most people simply aren't that (not in an Instagram/Love Island type of way, anyway). At the same time, global politics seems utterly nuts and people feel like they have minimal control over events.
But anyone can start a thread on Mumsnet, or Reddit, or wherever else. So for a certain kind of person, they come on and make something up, get a few responses, and feel like they've had attention and care paid to them for a while. Felt special. I think the sadness of it is that these people probably almost entirely lack feeling cared-for within their own lives. So they have to falsely generate it on the internet. Of course, some are just sympathy vampires, pure and simple.
I also think that sometimes, there's a kind of in-world celebrity baiting going on. Looking at MN specifically for a moment; some posters are well-known within the boundaries of MN as an environment. I've wondered if for a certain type of troll, there might be a warped kind of cachet to be had in posting their nonsense and getting a response from, say, AnyFucker, or WorraLiberty, or Datun. Or indeed your good self, Bertrand. I wonder if in their minds, it part-legitimises the troll post and therefore makes them feel like perhaps they're not actually a troll (even though deep down they know they are).