[Yosemitest]

Mechanical failure with gusting winds in a canyon is what I suspect.
Normally if the engine fails the blades will continue to rotate with the wind moving over them.
There's a procedure called "Auto-Rotation" that for emergency landings in such a situation with a pretty steep descent rate until enough speed for the rotors to slow the helicopter for landings obtained, but landing areas in the Grand Canyon are hard to come by, especially in the inner canyon.

Helicopters Won’t Just Drop Like A Rock If The Engine Dies ( 4:27 )


Autorotation - Landing a Helicopter without Engine Power ( 3:12 )

[InABunkerUnderSF]

Mechanical failure with gusting winds in a canyon is what I suspect.

That and failure to maintaining proper pitch angle during the decent in order to avoid canyon walls.

[redfreedom]

A pilot will likely correct me, but when in the Army, I was on a UH-1 that auto rotated to a safe landing. My understanding was the aircraft needed enough altitude so the controlled fall would get the main rotor going fast enough to build inertia so when the ground was approaching pitch could be pulled to soften the landing, if not make a perfect landing. My recollection was the pilot made a combination auto rotation and running landing.

Point is, if this aircraft did not have enough altitude to begin with, or a relatively smooth area to land, it’s toast.

[TaMoDee]

I fully agree with the auto-rotation. I should have used a better choice of words. In that terrain I’ll take a guess that the pilot was looking at that small gravel bar just to the right of the crash site as a possible landing site.
(Lost a very good friend in RVN when he had to go into the tree canopy when that was it. RIP brother!)