My guess is that leather had to have some sort of prevention of penetration of the dye. Such as it was oil tanned or treated withsome oil prior to dying. Plain ole veg tanned leather won't do that normally with black dye in my experience. I am not one to put neatsfoot oil on leather prior to dying. I treat the pieces after the dying process.
I have had rub off with using black before, but not completely rubbing off. USMC is the worst at causing rub off in my experience. Some hides of veg tan do differently with black. Sometimes i get more rub off than I should with regular Pro Dye Fiebing's black, I will go over with Angelus jet black and it reduces it. Sometimes I have used Show Brown over black to reduce rub off during buffing.
More info is need here from the OP to draw any more conclusions.