Badger, a blown tire like that is caused by the tire not being seated properly on the rim. Your tube issues sound like a sticky valve, which wouldn't cause the tire to blow off the rim. Overinflation of the tube could cause a blowout, but that doesn't seem to be the case here. If your tube has gone flat recently and been re-inflated, possible the tire bead didn't seat properly when you re-inflated. Maybe a bit of tube was caught between the tire bead and the rim. If the tube has remained fairly well-inflated for the last year, then the cause may forever remain a mystery, maybe a bad spot on the tire.

When you replace the tube, inspect the tire very well as you inflate it. It's possible that the tire bead was damage by the blowout and it may not hold properly on the rim again.