If I were a bettin' man, I'd go for the pinched tube. If the blowout was accompanied by a report (bang!) then it's either a tire failure or a pinched tube. Other flat modes may cause rapid deflation but rarely sound like gunshots.
Two tire failures can cause blowouts: cord failure, in which case the tire itself will have a gaping hole in it, or bead failure, in which case putting it back on the rim will be too easy and the tire will fit loosely.
A pinched tube is the result of improper installation, causing the tube to get caught (pinched) between the rim and the tire bead. One guy in my riding group had a slew of blowouts, which continued until a couple of us who knew what we were doing finally pushed him aside and made him watch while we replaced his tubes. Hopefully that's the end of the issue for him, until he replaces his tires at any rate!