Interesting comments.
The cause of the rapid pitch up will be the key: did he pitch up because of hydraulic failure, hydraulic hardover, something catching on the cyclic, a wire between the building, suddenly realising he wont fit between the buildings, or what? There is no yaw associated with the initial pitch up, so I would discount tail rotor malfunction or pedal hardover at the initial stages.
A Hydraulic failure looks like it could be realistic, but a Gazelle pilot needs to look at the aircraft jerky movements, and yaw, following the return from behind the buildings. Is it realistic that he would have let the aircraft sink into the trees just because fo hydraulic failure? What are Gazelles like after a hydraulics failure? Would a cavitating pump cause pitch up and yaw fluctuations? Any Gazelle pilots out there? I do note that Lovre seems to have inside knowledge that there was no such failure, but Crab's idea about the "Santa Factor" is a good one.
I cannot see retreating blade stall having anything to do with this. Firstly, forward speed is absent and so is excessive conning angle suggesting low RRPM or over pitching, and secondly, there is little if any roll associated. Lastly, when he had completed the 180 and returned into view, the aircraft should have been controllable.
Crab: the LZ is not where the aircraft crashed. He still had some distance to fly, and examining the footage does not reveal the pilot searching for the LZ at all, rather he is looking steadily forward and appears to be flying where he intended to be. Also, at the low airspeed, I am not convinved that there would have been too much cyclic climb resulting from the pitch up, hence no subsequent need to rapidly lower the collective, so I cannot see Vortex Ring State (even in it's incipient stages) in this one either. The aircraft re emerges from between the buildings without any descent, and little evidence of large conning angles suggestive of pulling max power against any downdraughts. I see your reasoning on trying to hover with insufficient power, but again, the conning angles just dont look too excessive - does the Gazelle bleed off quickly? Could he have suffered fenestron stall whilst trying to hover when he re emerges from the buildings?
As for the downdraughts, or turbulence, that may very well explain the pitch up, but why the subsequent crash? The footage I have seen shows the LZ, and there does not appear to be any excessive winds around the vicinity, including during any of the postcrash footage.
As for engine popping, that is a possibility but the popping noises were present before the aircraft arrives, and continue at the same intensity even though the aircraft goes out of sight between the buildings suggesting these were not related to the aircraft. Lastly on this one, wouldn’t the yaw have been in the opposite direction?
Regardless of why the aircraft pitched up, it appears that there is some sort of loss of control issue after that manoeuvre, and this must be control integrity related either due to failure, insufficient power/fenestron authority, or object strike – so my guess is the ultimate cause will be whatever caused the pitch up.