Musicians with Dystonia Bulletin Board

There's no good reason why it would or should be involved, but that's FD for you. I think there are other reasons why a player may develop this to some degree even without FD.

1. There was a famous picture of Segovia's RH from the player's point of view, with the fingers pretty well squeezed together "like a bunch of bananas". Nothing wrong with that, BUT if you consciously try to adduct (pull the fingers toward the center) you MIGHT also fire the opposers, especially if you're trying to hold in the pinkie. So the trick is to know the difference in feel between just adducting (generally a good thing) and engaging the opposers.

2. There seems to be a bit of a natural inclination for the thumb and pinkie sides to fire together. (Almost too obvious from "opposable thumb" feature). So if you develop tension in the base of the thumb (perhaps from trying to keep it close to the hand, perhaps from a random FD signal), there's a good chance the pinkie side will fire too. And vice versa.
Clearly if your starting posture includes this at all, you're going to have problems.

Leisner mentioned in one of his videos how important it is to keep the pad at the base of the thumb (thenar eminence) free of tension, and I think he's absolutely right. I think the potential of bringing in the opposer muscles is part of that.

The trick is, if your hand is "cupping", it's not easy to fix. You have to find a way to get it to let go. The key is that deeper level of perception to know if it's happening (watching for the line of knuckles curving is a clue), and gaining the sensation to release it, and never let it creep in. Regular good, safe stretching of the base of the thumb is important.
There are targeted tactics for helping with this if you know someone who needs it, but that gets into details that you probably don't even want to hear about unless you have to.