The helicopter pilot reported that, while transporting snow skiers, he landed on a 20º slope below the mountain peak, where he directed the guide and two skiers to unload with the engine running. The pilot saw loose rocks from the peak above fall into the main rotor disk, and after the skiers were clear of the helicopter, he raised the collective and brought the helicopter to a hover. He added that, while hovering, he did not feel any controllability issues, so he repositioned the helicopter up the mountain about 30 ft and "toed in" to unload additional skiers. After the remaining skiers unloaded, the pilot started flying the helicopter down the mountain so that he could shut down the engine and examine the main rotor blades. The pilot added that, about 10 seconds later, the helicopter began to shake violently. He initiated a forced landing, but about 150 ft above the trees, the "flight controls became progressively hard to control," and the helicopter pitched up. Immediately, he lowered the nose and collective, and about 30 ft above the snow-covered ground, the main rotor disk struck trees, and the helicopter subsequently impacted terrain. The helicopter sustained substantial damage to the main rotor blades, fuselage, tailboom, and tail rotor blades. The director of operations reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.