Tire Sidewall problems of the 2004 Chevrolet Avalanche

Tire brand and size is cooper discoverer stt 33×12. 50r20lt. I was coming home on my lunch break and it blew out I didnt hit anything in the road or anything I have another tire the same brand and it started to separate on sidewall also but I felt a vibration so I took the tire off and put my other spare on and the vibration stopped.

: the contact stated that while driving at 70 mph, there was a loud vibration coming from the left front tire. Upon immediate inspection, nothing was observed. The vehicle was then taken to a service dealer, where it was determined that the tread on the tire had separated. The tread was wearing unevenly and the sidewall was splitting. Updated 8/21/2006 -.