The likely cause is that the yellow ink was made with diarylide pigment. Diarylide is a brilliant yellow pigment that has poor fade resistance from the UV rays in sunlight. Ink pigments will eventually fade when exposed to sunlight; how rapidly they fade depends on the pigment (yellow and magenta pigments fade fastest) and the amount of exposure (heat and humidity also play a role). Make sure to find out how customers will use printed pieces and, in cases of significant exposure to sunlight, use fade resistant inks and a varnish or lamination with UV inhibitors.