Why Did My Carpet Turn Yellow Beneath My Area Rug?

In my 13 years as a carpet cleaning professional in the Twin Cities area of Minnesota, I have run into many cases where the carpet underneath an area rug has a yellow tint to it.  This discoloration is literally the size and shape of the area rug that was sitting on top of it.  It took some time but through my research I learned that the gases released from the glue that holds the carpet together can actually change the ph. level of the carpet.  This usually takes months of trapped gas to make this happen.  I would imagine humidity and airflow as well as other factors, contribute to this phenomenon.  Unfortunately there is no known method to reverse this problem.