Why do the headlight lenses become hazed, yellowed, or pitted?

Hazing, yellowing, and pitting is the result of several factors including sunlight/UV exposure,
impact with sand and road debris, climate/environmental conditions, as well as age, composition,
and design of the headlight lens. The yellowing you see is either a degraded surface of
polycarbonate or a degraded UV coating or thermal cap layer. In any case, it needs to be
removed to restore clarity to the lens.