The frame rate isn't high enough to tell for sure, but it looks like the initial explosion might be between the strongback and the second stage, followed less t

The frame rate isn't high enough to tell for sure, but it looks like the initial explosion might be between the strongback and the second stage, followed less than 1/30 second later by the second stage exploding. Certainly the initial explosion spreads out vertically much more than horizontally in the first 1/30 second, perhaps implying that the explosion was in vapour outside the second stage.