Q: Yellowing of power cable under organizational tab

Hello Macbook Pro discussions,

     I recently went to the Apple Store for other reasons, but I mentioned to the person I was talking to that underneath the organisational tab that I had kept in the same place for a long time the power cord became yellow. It was not a big deal to me that a tiny bit of my wire was yellow, but because I was under warranty on my laptop, they replaced it. I just wondered if that was normal for an almost year old power cable, if it was caused by excess heat or something else preventable, or if it was a sign of a faulty power supply that could cause problems in the future?

Thanks ~ Bryce