UV light causes the yellowing, often it's sunlight, but even certain compact fluorescent bulbs and halogens can accelerate the issue. Generally speaking, the resins do not have proper UV inhibitors mixed in during manufacture.
Packaging is as susceptible as plastic parts.
The problem can be reversed to some extent, see the fine work by Chansformers and others here:
http://www.cybertron.ca/boards/showt...rogen+peroxidehttp://www.cybertron.ca/boards/showt...rogen+peroxide
The same process applied to parts could be applied to the PVC bubbles
