First reports said they "ran out if air".
Helicopters don't "stall" in the conventional sense.
Could have been something called "settling with power" where the free air flow into the top of the rotor disk is replaced with descending air from the rotor downwash. This descending air is re-ingested during a high rate vertical descent and causes loss of lift due to a change of the angle of attack of the relative air that the rotor blades "see".
This analysis is based upon a twenty year old memory of a tricky-to-understand aerodynamic concept. Please correct or polish as necessary.
The photos we are seeing here are of the aircraft AFTER the US team intentionally blew it up so any photo analysis by us "experts" will probably be wrong.
Regardless, outstanding job by an incredible team under extremely difficult circumstances.
EW