Is sun the only cause of this

It might be hard to see but its clear in person my nrg zane has yellowed. Whats weird to me is all these figures where sitting on my shelf in the same spot last year until I put them into storage and only took them out now. Now I bought zane, kai and Jay used so I dont know how long they were previously in the sun. But when I put the figures into storage they were normal and now zanes like this. What im ultimately getting at is there a way to stop this or is it a fate I have to deal with and...