What triggers this, does anyone know?

Two (same) horses in the same spot at the same time. Corrupt save or corrupt installation.

You again? Do you have anything else to say other than "corrupt installation" or "uninstall mods"?

Ha, somehow I just knew you would reject the truth. Idiot.

I don't know what you are trying to achieve by continually making a complete ass of yourself. You do know how to hold a grudge, though, I'll give you that.

As some one else said, it's two horses standing on the same spot. I don't think it's a corrupt file, though. I think it happens sometimes when you have the population slider (can't remember what it called?) turned way up. I keep mine high-- sure, you get two-headed horses and sometimes two people standing inside one another sometimes but I can accept that for the livelier crowds.

And by "sometimes" I really mean some spots.

To this 'hickory' thing, everything is a corrupted file; the thing doesn't have any other response, so it trolls posts with his shilling, the simpleton.

Anyway, I took the population density to high and it still happens. What clears the issue, for a bit, is resting. When you rest/meditate, the problem fixes itself, but alas, as soon as you leave the area where the horses are and return, they glitch out again.
It's the most noticeable at the Niflgardian camp where you have "Witcher Gear Cat upgrade 1" treasure quest.

#cd projekt red can't fix anything.