What causes soft foam liners to automatically degrade?

Soft foam liners in helmets, earmuffs, and even inside shoes will degrade and just break down into little pieces (when touched) after being stored for relatively long periods of time - such as 24 to 36 months (even when stored in the darkness). This happens even for new unused items. But interestingly - using those items regularly either prevents such breakdown or at least significantly slows down the degradation process. For example, the foam inside soles of sports jogger shoes can last for a long time if the user wears the shoes regularly, but the foam can disintegrate by itself (self destruct) if the shoes are left unused for 2-3 years. Anybody know what are the mechanisms behind this degradation? Why can't those companies produce a stable foam material?