The worst thing of owning a nice car, every week you find some little chip/crack/scratch!
Its not UV damage for sure
Forensic analysis:
Im gonna say that because the part is manufactured through injection moulding it can be seen that the cracks show the direction of the polymer melt direction. Chain allignment has caused anisotropic (strong in one direction, hash in another) properties. The headlights have had a good hitting my something and causing the cracks to nucleate between the longitudinal flows and propagate into the long lines you now see. Further stress e.g from vibrations e.g driving has caused the lines to get long and wider, hence more noticeable.
Will need replacing.