Everything from commercialism and employment to attitudes towards nakedness and violence.
Because different languages have different pronouns. You don't adopt function words into your language the same way you can adopt content words (unless you're, for example, code switching, but even then you don't switch for a single function word).
(Also, Old Norse was quite a bit closer to Old English than presumably modern English is to some of those languages in question - Finnish, especially.)
Social memia is the game of acquiring cheap prestige. Imitation of fleeting English fads is one such cheap use.
Could it be these Finns interact with English speaking tweeps?
Care to expand?
First, yes, I made a typo and meant to say "foot binding".
The reason I mentioned this is because foot binding originally began as a cultural practice among a small group of people in the 10th century. By the time of the Qing dynasty (1644-1912), most social classes were practicing foot binding.
Thus, what I'm trying to say is that the Finns are adopting a cultural practice (pronouns in their twitter bio) as a result of being exposed to another group of people doing it.
The reasons for why they chose to adopt it, though....
I have no problem being on friendly terms with people who think it's a bit silly of me to have them there. But people who are so offended they don't want to follow me? Good riddance.
If people just disagree and find it silly (and I did myself at one point), then that's their business.
But using someones preferred pronoun is no different from trying to get their name right. It's basic respect and courtesy. That so many people take offence at being asked to show basic respect for others is the reason why so many people who have no personal use for putting our pronouns anywhere increasingly do so, to help normalise it.
People don't need to like that.
But if that's enough to fuel a fire, then let it burn.