It could have been just a bad seam in the tube. Also I always powder the inside of the tire to prevent the tube from sticking is some areas. After pumping up the tire a place where the tube might have stuck and then snapped loose could have caused your problem. I powder the tire, and after pumping the tire up after a patch or a new tube, I let most of the air out and then pump up again. That lets the tube seat itself properly.