To go vertically into the ground like that there is only one realistic cause.
The aircraft stopped flying! stalled.
Whatever lead up to it stopping flying is the questionable bit.
Hence why it is so important in any scenario to keep the wing flying ie trade altitude for energy.If assymetric and loosing control pull both back and keep flying.
Better to crash in control than to crash out of control
Pace
Assuming of course he was not already incapacitated as I suggested in post 11, then no amount of energy is of any use.