They are UV light waves and ozone. Both of these attack the long hydrocarbon chains of the rubber and by breaking these chemical bonds, shorten the molecules with resulting loss of elasticity and other problems.
Tire manufacturers add two primary sacrificial protectants to the rubber.
To protect against UV, they add carbon black. This is why tires don't come in designer colors to match your paint. Thus the basis of rubber parts turning gray as they age. To protect against ozone, tire manufacturers add a wax based sacrificial protectant. The ozone attacks the wax and depletes it.