Occasionally, dyes used on leather rub off, or migrate, to other materials. This frequently happens with suede and nubuck leather because these types are sanded and buffed and release excess dust on the surface. This dust can't be completely removed during the process so rubbing it with another fabric, perhaps clothing can cause color migration to the other material.
Additionally, leather that is oiled or waxed with a pigmented dye can also transfer color when they come in contact with heat which causes the pigment to loosen and transfer.
From time to time, dye from the back of the natural grain leathers may also penetrate the surface and leave blemishes. This has been known to happen when markings placed on the animal hide to guide the cutting process penetrates the hide and leave visible pigment on the surface.
Finally, it can occur from adhesives and insoluble dyes that penetrate the leather over time as a part of the aging process. The most frequent cause for this is poorly treated, low quality, and leather substitutes.