Thinking that for whatever reason the drivetrain was disrupted so no power getting to the rotors. If the drivetrain was disrupted enough it could cause misalignment or unseating of the swashplate/pitch control mechanism. This could mean that the aircraft was unable to enter self-sustaining autorotation and therefore the flight turned into a race between an emergency descent and a steadily decaying MR rpm. Realising that the MR rpm is now way below min. auto rpm, and the aircraft is descending very rapidly towards the roof, the pilot makes a last heroic attempt to level the aircraft and pull whatever meagre amount of pitch is left. sadly not enough and the aircraft falls through with stopped rotors?