[danbruc]

On the video it looked to me like a tank ruptured somewhere in the top half of the rocket, probably the second stage, and set free all the fuel. The rocket kept its course for some time until it suddenly disintegrated.

[phkahler]

I was impressed with how long the first stage kept going with all that happening up above.

[n_coats]

maybe the rupture was the initial bit of smoke (it looked the larger of the two from my view).