Why do the boys run from the dead parachutist?

When Ralph, Roger and Jack climb the mountain they see the dead body of the pilot. The parachute lines have become tangled in the rocks making the body move when the wind blows. This creates the illusion that the movements are being made by something which is alive. The parachutist's body descended to the mountain top during the night and none of the boys witnessed its arrival, so they had no reason to think that it might be a parachutists body. In the darkness the boys assume that the indistinct shape which they see moving must be "the beast." Sam n Eric also came to the same conclusion when they earlier saw the dead pilots body, aided also by the talk of "beasts" and "ghosts" at the meetings.