<amateur detective mode>
From the available videos, it seems that the rocket had 3 pitch overs - each one larger than the previous one, and it seems that the rocket was in an excessive roll right after liftoff that continues to accelerate till breaking up.
I have a hypothesis that was first mentioned by a Russian forum member
here:
1. For some reason the Proton started to roll excessively within a few seconds of liftoff - reasons not yet known as this point.
I agree with this.
2. The excessive roll caused the rocket to start to pitch over towards a certain direction due to torque from the angular acceleration (remember tau = I * alpha?) - it may have been magnified by the initial pitch over command.
Agree with this also.
3. Sensing that the rocket is pitching over limits, it attempts to gimbal some of the engines towards the other direction, but since the roll is accelerating (and may already be out of control by then), it over-corrects in the opposite direction and caused the rocket to lose pitch (and almost certainly yaw) control.
Pausing the 'Apocolypse in HD' video at around the :29 - :30 mark seems to support this (happened to spot this accidentally, as YT paused to continue loading the 1080P video).
4. By then the worsening roll puts the rocket in a positive feed-back mechanism - the more the rocket tries to correct the pitch, the more it over-pitches in the other direction. This third pitch over finally sends the rocket horizontally and points it towards the ground. The rocket continues to roll crazily until it breaks up - heads first (tau = r * F) and then the whole rocket as the fuel tanks blasted open. Then it impacted on the ground.
Any comments on my thoughts?
</amateur detective mode>