A sidewall bubble is a bulge protruding from the sidewall of the tire. It is caused by air leaking from the inside of the tire into the carcass or body of the tire. Most tire bubbles are caused by impact damage. The impact can also damage the sidewall cords and weaken the tire causing a tire blowout. Did you recently hit a pothole while you was driving? If so this may be the cause of your Tire bubble. Driving on a flat tire for a short period of time can also cause a tire bubble to form.