More specifically, Samneric see the dead parachutist bowing in the wind. They interpret this as the beast, and immediately take off down the mountain to alert the others.

Sam and Eric are young, and twins and alone. There is a special bond with twins, just as there is a special bond among the "collective common man. Sam and Eric, in the LOF microcosm, represent this collective common man, who perceives danger, but feels powerless to do anything about it without specific direction. Of course, the specific direction can come from two places: Good, represented by Ralph, and Evil, represented by Jack and his more evil sidekick, Roger. So, here are young Samneric, alone, deserted, afraid as many people find themselves in various situations in life and they react symbolically as anyone in the same situation in real life would act. They are uncertain of their station, uncertain of their leaders, and uncertain of their future. So, quite simply, they are afraid. Golding intended this in his powerful novel about the fallacies of the concept of Utopia. The specific fallacy, here is that if all parties are basically "good", then only good can result. What Samneric or Sam and Eric have just experienced is that this is not true, hence the fear.