Just glad you are ok!
After looking at the way that barrel split at the lugs, I'd have to agree with the squib followed up by another round (perhaps in a rapid fire situation the squib wasn't detected since Glocks have short trigger resets and fire very quickly).
With my single stage press, I've never had a squib or double load of powder. I always fill 25 rounds at a time and visually check the levels before moving to the press. In my Dillon Sq. Deal, I've had issues with misfed primers, or not aligning the empty brass at the 1st station properly, where the press then indexed without filling the 2nd stage. A quick look in the case at the 3rd stage will catch it. It's easy on progressive loaders to get relaxed, but I have a habit of a quick look inside the case before placing the bullet on top.
NRA Life Member