Video Caption: A more detailed analysis of the spacex falcon 9 rocket explosion.
Several interesting details. Firstly the main explosion is actually from the fuel from the second stage and the liquid oxygen from the first stage.
In order to save weight on the rocket the second stage of the falcon 9 uses a common wall for the liquid oxygen and fuel tank (rp1). Any rupture in this tank wall would doom the rocket.
The quiet ‘pop’ may well be a failure of a helium tank. They are usually used to keep a pressure in the tanks while they are emptying due to the rocket burning the fuel. If one of these had ruptured while the oxygen tank was full, it could have overpressurized the tank causing it to also fail.