Well, this isn’t so good. It’s not the usual sort of explosion, where there’s a very sudden fireball and the vehicle turns into confetti in a split second; this disaster seems to be stretched out over a few seconds. It kinda looks like there was a fuel or oxidizer dump from up front… perhaps the second stage. Also early on in the “anomaly” you can see something drop away from the vehicle. I wonder if perhaps that’s the Dragon capsule? The disaster was good and slow… *perhaps* the abort systems got the capsule away. But I’d imagine if that was the case the booster itself would have *promptly* turned into so much tinfoil.
UPDATE: A tweet from Elon Musk says that there was an “overpressure event” in the second stage LOX tank. Cause was “counterintuitive.”