I am reading that wrong, or is Elon Musk's new tweet suggesting it was the second stage that blew the whole thing up, while the first stage was flying?
"There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause."
That sounds weird.
That's exactly what the video shows. First stage was running fine, but the second stage began to violently outgas above it during the flight. At that point, the nearly-empty first stage simply disintegrates.
it also looks like the capsule separated (or was blown away) from the upper stage during the explosion.