Something similar happened to me a couple weeks ago - I believe it was due to some of the tube being pinched under the bead of the tire, specifically near the valve, where it can be hard to seat the tube fully inside the tire (depending on the tube). The tire was fine for a couple weeks, then one morning I went out to the garage and the sidewall of the tire had blown out - separated from the bead. I guess the strange pressure and forces just finally got to it. Oddly enough, it happened overnight, and the tube didn't burst - it was just swelling out the side of the tire. Not sure if this is what happened to you, but its one thing that can happen.