The main culprit is the light, but other ways are possible to wreck the plastics. The energy brought by the light cuts the polymeric molecules when the right wave length matches the energy activation level of a particular link betwenen two atomes. Polyethylene family is very sensitive, polyester far less (but still). It's the same phenomenon with the fading of the varnish and the paint.
The molecules become gradually shorter. The material looses its overall strength and suppleness/elasticity, finally becomes brittle or even falls apart in tiny bits/dust. It's annoying for the garden table or the beatifull paint job on the truck, but for a PPE, that really sucks.
With plenty of pigments in the polymer, the ligth is stopped very soon under the surface and converted to heat, loosing its capability to cut the links. Each particule of pigment acts as a shield and protects the polymer just under it. Black or aluminum powder are the best for longevity.
Try to press or fold the piece of plastic. Usually, nothing happens. But with a weakened polymer, you can hear a distinct sound when the exposed surface is wrecked by thousands of microscopic cracks. Eventually not so microscopic.