It's due to overuse of them, especially repetitive repeated motions--a la the usage of them when playing video games.
I pull my keyboard stand out a bit as to give my hand the best positioning possible (well, I do this when I notice the issue starting, to help prevent it or rather to to at least delay it), I use a gaming mouse, I suspect that the gloves I wear might also help some, so I do a lot to try and prevent it.
But when you play a game for four hours and the game requires repetitive and rapid back and forth wrist movements using your mouse to rapidly dash from one side to the other side and back with occasional clicks and there's no interruptions, the stress builds up.
And once it builds up, it can take days to actually heal fully.
Kinda sucks, because I was having fun. The Kongregate badge of the day for Friday (which is tomorrow, but as it's technically past midnight, I can earn it today) is for a game that I never discovered until now, a game called Tesla8. The thing that makes it so amazingly addicting is that it basically is combining the mechanics of Breakout (one of my favorite games of all time, at least one variant of it, which I have tremendous nostalgia for and which I had to have logged like 50-200 hours in), in that you're breaking blocks with balls you bounce, with what amounts to a top-down (I think that's the term for it, as in, not side-scrolling from left to right or right to left, with enemies appearing from the top and you at the bottom) ship shooting game that's partially bullet hell but also combines the space invader / turret defense mechanic of if one of the blocks getting through, you die, but allowing you to upgrade.
I'm strong enough right now to semi-reliably get to the boss and in theory fight the boss, but I was in the process of grinding out for some extras. (I want to get things maxed out or close to.) Sadly, my wrist injury kicked in from this strain so I had to stop.
It's a bummer, but that's the consequences of not being young while also being a gamer. Repetitive usage of your hand is legit something that does indeed inhibit gamers, even professional ones. You literally have people who play video games for a living end up with the type of injury I have, and have medically mandated breaks for it specifically due to how much damage it can cause.