Every environment is different so unless you do a lab test on your particular parts the reason the failed is just a gigantic guessing game.

Being from the plastics industry (20+ years) and being VERY familiar with Urethanes as the company I worked for made all of the Urethane ski goggles for both Scott & Smith Goggle (my name is just a coincidence ) I can tell from personal experience having been involved with countless lab tests, that UV is the biggest enemy of Urethane, ozone, etc. are also problematic.