I don’t see any signs of the second stage igniting before separation. Instead, the upper part of the vehicle appears to be rapidly venting a gas for about 8 seconds before the rocket broke up. My wild guest is the second stage’s liquid oxygen tank ruptured.
I’ll follow up with a second wild guest - the relief value for the LOX tank failed leading to a rupture of the tank.
Elon tweeted this:
Elon Musk @elonmusk 6m6 minutes ago
“There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause.”