Based on my own limited knowledge of rocket engineering, here’s my analysis of the Antares rocket failure. Essentially I have watched HD videos of the launch repeatedly and I have come to three possible reasons for the failure. The Antares rocket has two rocket engines on its first stage which are fueled by liquid oxygen and a refined form of kerosene known as RP-1.
THEORY A. After liftoff you see a brightening of one of the engines, almost as if one engine was being over-throttled. This would have resulted in the engine exploding from an overheat condition.
THEORY B. When you see the brightening of the engine this could be a failure of the engine’s thrust chamber (the bell on the end of the engine). A micro fracture or a flaw of some kind may have been present in the thrust chamber and without specific equipment to inspect it, you’d never see it visually. In this scenario the engine bell failed and caused an overheating situation in the aft section of the rocket and/or damaged the remaining engine resulting in that engine exploding from overheating as hot gasses escaped from the failed engine.
THEORY C. There could have been combustion instability in the thrust chamber leading to very high stress levels placed on it, resulting in an exploding engine.
Once the engines failed, lift was cancelled and gravity took over. The rocket fell to the ground, the liquid oxygen and RP-1 combined and exploded – destroying what was left of the rocket and most of the launch-pad. If you look very closely at the launch video – you will see that only one engine actually brightens, it looked to be the engine on the left side.
For reference here is a diagram of the Antares rocket and one of the better videos of the failure: