Why did my tire fail?
Two common possibilities:


Your brake pad is contacting the sidewall of the tire. Classic cause for a blowout.
You've used something like heavy-duty engine degreaser to clean your drive train. This will chemically change the rubber and it will remain "tacky" forever. The best degreaser, that won't damage rubber is kerosene+lemon juice (or if you want to spend money, prepackaged citrus degreaser).