“Wind burn” when drying veg tan leather

[Mel42]

anyone have experience with veg tan leather discoloration from exposure to moving air while drying?
 

I’ve been working with a very pale veg tanned leather, wet-stretching it around stool cushions. I have a sort of “drying box” with a fan that I put the cushions in to dry to protect them from dust and such in my work space. I’ve noticed that when placed too close to the fan or a small gap inside the box, the leather can develop a dark discoloration as though a wind burn. Anyone know why this happens or how to better prevent it? Thanks!