Heli hunting

could cause both. if the pilot has head up forth point of contact and hits the trees, it could do a sudden stoppage on the rotor system, which it comes down or the trees bust apart the rotor blades, it agains falls out of the sky. google blackhawk crashes into pine tree or something of that effect and it shows a 60 coming in hot, lands, bounces forward and cuts down a 6-8" pine top, and if its the original vid, it shows the damage after the rotors stop turning. the entire drivetrain has be gone through and a lot of chit replaced because he was show boating at some kind special event.
now, as far as trees disrupting the airflow, yes, its called settling with power. you have full power, but because you are recirculating "dirty air" your own vortices back into the rotor system, the blades eventually stall out bc they don't have "clean" semi-undisturbed air recalculating back into the rotors.