The outer layer of the BvS suit is made out of thin vinyl, and thin vinyl and faux leather (pleather) that many manufacturers of this hobby tend to use are not meant to last.
As for what's causing it, humidity and the leaking plastisizers in the figure can be a huge factor. Other than that, sunlight, moisture and even the natural oils in your hands can contribute to deterioration.
The best you can do it to treat any thin vinyl or pleather with stuff like 303 Aerospace or car interior products, keep it away from humidity and NEVER EVER keep mixed media figures in box, as a few posters before me have said, if a 1/6 mixed media figure is "brand new" and has been in storage for years, you'd be better off getting a used one instead.