Certain plastics degrade faster when in contact with other classes of plastics (or the chemicals in other materials)... and Fort Max is one that has a fair bit of it in direct, prolonged contact with styrofoam. I wouldn't know for sure if the styrofoam is doing it, but if temperature, humidity, dust and UV aren't factors, the contact with other materials is one to consider.
But as noted above, eventually, all Fort Maxes will degrade & discolour... some will just do it faster than others. They were produced a quarter of a century ago... and weren't designed to last forever.