I'm not sure as to if he pulled the nose up to avoid colliding with the buildings or if he had the nose up and got a windy surprise. Anyway, getting down he seems to have had too little speed, getting caught by its own downwash, in what's called a
vortex ring condition. Something currently not modelled in FlightGear.
A vortex ring condition will make you loose altitude
fast. It's most dangerous when descending at slow or no airspeed, e.g. while landing. It is more or less the helicopter equivalent of a stall. As with fixed wing aircraft forward speed is crucial for the recovery.
Have a look at
Howto: Fly a helicopter#Preface on the FlightGear Wiki.
An interesting YouTube clip explaining what it is in a practical
(but slow) way is this one (10 minutes from a flying lesson):