This is common in antique rugs. The dye used to colour the yarn weakens the fibre. With rugs it is nearly always the black that goes first. It puzzled me why the effect was often very patchy. One end of the rug the black was fine. The other, the black corroded away completely. I went on a trip to Iran and visiting a weavers house I made a discovery. Any unused wool that was left over was wasted because it would not match the next dye batch. To save money they would dye the waste wool black. The double dying and in particular the iron oxide used in the process causes the weakening of the wool.
The effect can be quite attractive in an old rug. In fact, if you look at new Turkish rug the effect is often imitated with the black wool being cut lower than the rest of the pile
Al