Development of a yellow cast in aging paint; most noticeable in the dried films of white paints or clear varnishes.
Oxidation of alkyd or oil-based paint or varnish.
Heat from stoves, radiators and heating ducts.
Lack of light (e.g., behind pictures or appliances, inside closets, etc.).
Top quality latex paints do not tend to yellow, nor does non-yellowing varnish. Alkyd paints, because of their curing mechanism, do tend to yellow, particularly in areas that are protected from sunlight.
(image courtesy of The Rohm and Haas Paint Quality Institute)