Its certainly an unusual name, and an unusual phenomena - but it happens quite a lot with carpets and/or rugs. But what is it? If you have suffered from this (or simply noticed it), and wondered what it was, then I will do my best to explain it as it should hopefully shed some light on it.
How is it noticed?
First of all it shouldn't be confused with the original carpet colour simply being retained under a rug (which of course does happen a lot) as the yellowing can happen on any colour of carpet. Lets presume you have a light grey carpet, and you lift the rug up - and discover some yellowing - this would be a classic case of phenolic yellowing.
Why does it happen?
The primary reason is due to what we professionals call 'off Gassing'. Essentially (in the case of the carpet) butylhydroxyltoluene (BHT) is used as a preservative either within the carpet, especially the latex on tufted carpets, and also some underlay. When a rug is placed on such a carpet it seals in the area like a lid 'trapping' air and causing the carpet to simply not 'breathe'. It's this trapping that contains the chemical that would normally float in to the atmosphere and so subsequently reacts and discolours the carpet fibres.
If the carpet was not a tufted pile (or a carpet not containing BHT) and there is still yellowing under a rug, then you may need to look at the rug in this case as the primary cause. It will probably be more than likely a secondary backed latex glued back (often has a smooth textured backing that has no resemblance to the pile of the rug), and this could be the reason why it's caused, due to BHT in the backing compound. It's basically the same as the carpet scenario above - but in reverse. The 'off gassing' is trying to escape from the rug, but is being trapped by the carpet - causing....yellowing.
Can it be cleaned away?
Essentially no. It's permanant.
Technical Definition:
Phenolic yellowing is caused due to the presence of phenolic compounds on the textile material, reacting with the oxides of nitrogen in an alkaline medium. The phenomenon of phenolic yellowing is associated with the storage of finished textile material, packed in polyethylene/aromatic polymer material or cardboard cartons.
Aromatic amines (PPD-Para Phenylene Di-Amine) and phenolic compounds (BHT-Butylated Hydroxyl Toluene) are increasingly used as anti-oxidants and stabilizers in organic polymer packaging materials, lubricants and foam. These and the phenolic derivative from the lignin in cardboard form the yellowing precursors.
Author: Kevin Loomes
http://clean-pro.co.uk/Carpet_Cleaning_Blog.htm#Phenolic_Yellowing
