The Trousers I Bought From Bershka Turned Yellow

I went to Bershka about four months ago to buy a pair of pants for myself. Initially, I didn't think anything was wrong with them, but just recently I noticed a yellowing around the knee area. When I wore them out a few days ago, there was no problem; however, when I got home and examined them, I noticed that the color had faded. I was hoping that washing them would solve the issue, so I made the decision to do so. However, it was to no avail. Given that I did not expose them to direct sunlight, I find it hard to believe that the problem lies with me. Do the products of this brand have a known quality issue, or did I just have a string of bad luck?