This photo comes from the Subaru subreddit of Reddit.com. Member STiFTW posted these pictures after a track day at Mid American Motorplex in Pacific Junction, IA. The boots for the caliper piston got a little toasty after a day of hard braking.
While this failure is extreme, it still is happening everyday to the vehicle you are driving. Every time you brake, heat is generated by the brakes. The heat may not cause the boots to catch fire, but it does degrade the boots little by little on a molecular level, call it oxidation and good old entropy. Every heat cycle causes the boot degrade and break down. Eventually, the boot will become stiff and crack.
One way to delay the ravages of time and heat is to use a proper shim on the pad and piston. Piston you say? Yes, there are now shims for the pistons that reduce noise and prevent heat from being transferred to the piston and boot. NRS Brakes brake pad sets come with a shim that inserts into the piston to prevent heat from being transferred into the pistons and brake fluid. come with acts as a barrier, minimizing heat transfer to the brake fluid. Coupled with a brake pad shim, it can lower temperatures.