All the finest minds in the helicopter world looking at the same high quality video and we can't agree whether it is settling with power (insufficient power to stay in the air) or Vortex Ring State (VRS, recirculation of air..).
My opinion is that it was VRS. Judging from the smoke afterwards, the helicopter flying backwards was going downwind, then it pitched forward to stop (but still going backward in relative air) and the crew lowered collective to descend. Classic VRS entry conditions. You can see the aircraft accelerate downward (ROD looked VRS to me, don't forget it was only up about 300' at the time).
I'm ruling out settling with power simply from what I know of the performance of an empty Mi-8 with low fuel, three crew only on board, at sea level. I just can't believe they would have had any shortage of power to cause them to fall out of the sky. Perhaps a Mi-8 pilot can verify. Light weight is also a factor in VRS entry condition.