The plastic is reacting to both air and sunlight. Many plastic machines and cases have yellowed over the years. It's a common complaint. My Janome 9000 case is really yellow. The machine is fine because it's stored in the case. There is a hydrogen peroxide mixture that is available to purchase or to make on your own. Computers have had the same issue. If you google Retrobright or yellowing plastic there are quite a few sites and recipes. I've never tried it, so I can't confirm that it works, but it sure looks like it does if you want to take the time to try it.