It's being ignored by most in this thread because the mechanism for the failure was completely different. In your case the tire sidewall failed - that indicates either than something cut the tire (and that's a possibility even if you didn't notice it at the time) or the tire was defective and had weak cords in the sidewall that failed under the stress of the tube's pressure. But the OP's tire remained intact. In his case, the tire bead first started to separate from the rim (which is when he noticed an unusual sound prior to the blowout) and that allowed a portion of the tube to escape from under the tire bead and then suffer the blowout. The most common cause of this type of failure is improper installation with a bit of the tube being trapped between the rim and the tire bead. That pushes the bead up and off the rim and eventually lets part of the tube escape. But it could also be a rim and tire combination that's incompatible or a defect in either the rim or the tire.