Any answer given would just be scratching the surface really as there are A TON of factors to be considered, so it's a bit...difficult to really answer this. But I guess to give super basic answer: you get stalked/stalk someone simply because -------. No I did not censor anything: you could replace the ------- with pretty much anything you can think of, even something minor, and it can be someone's reason to stalk someone / get stalked. For example: you bump into someone at a grocery store, they drop something because of it, you pick it up for them and hand it back and then leave. Sounds like you just being a nice person since the bump is what caused them to drop said something and you just picked it up for them, what's the deal? To them, however: that could unironically be a reason for them to start stalking you. From taking revenge, to being para-social, to having mental health issues, to wanting to be controlling, to having low self-esteem, and many, many more reasons: if you can think it, then it can be a legitimate reason to stalk someone / get stalked, both IRL and online.
I won't pretend to know the RP community for FF14 that well since I keep to myself, but I have no doubt that like other RP communities; there are absolutely a few who take it WAY too seriously, and just like all the reasons listed above, anything you can think of in-game can be a reason for them to stalk you / get stalked.
"They said I was bad at RP!"
"They didn't like my glamour outfit!"
"They undercut me on the marketboard!" <- [This one here is why this 3rd party program actually exists apparently: because the maker wanted to know who the retainers of the person undercutting them on the marketboard belonged to.]
Overall, to put as absolutely basic and blunt as possible: People stalk and get stalked simply because. Probably not a good answer as a whole since answering "How does someone get stalked / why does someone want to stalk?" is pretty difficult due to all the factors and parts that make up each specific stalking situation, but hopefully it offers a bit of insight into it.