It is due to direct sun contact, however, I put a white bodied Hoover Junior in the loft a few years ago and it has yellowed terribly, it is cery dark up there, no windows or anything, pitch black, I think extreme summer heat and freezing cold winter tenperatures have an affect of the pigment of the plastic.
I love the new smell of a vacuum when you turn it on, surprising it still smells freshly new and not musty, as it will have been stored for 26 years.
Alex.