Why do Piggy and Ralph join the dance at Jack's feast?

Piggy and Ralph join the dance in Lord of the Flies because hunger, fear, and the pressure of the mob overwhelm the civilized self-control they usually try to represent.

They go to Jack’s feast in the first place because the promise of meat is powerful. Ralph mocks Jack’s group for acting like a tribe, but Piggy wants the food and covers that desire with a sensible-sounding excuse: they should go “to make sure nothing happens.” Piggy usually stands for reason, rules, and civilization, yet his appetite draws him toward Jack’s savage world.

Once they are at the feast, the storm frightens everyone. Ralph laughs that Jack’s tribe has failed to build shelters, essentially mocking them for not being practical or civilized. Jack answers by leading “their dance,” giving the boys a way to turn fear into group violence. The circle chants, “Kill the beast! Cut his throat!” and the group becomes a frenzied mob. In that moment, Ralph and Piggy are pulled in with everyone else. Even they press forward as the dance takes over.

Their joining the dance shows that Ralph and Piggy are not immune to the same darkness affecting Jack’s hunters. Ralph usually works toward order and eventual rescue, and Piggy believes in law and reason, but both still share the fear and savage impulses that the dance releases. When Simon comes out of the forest with the truth about the dead parachutist, the dancing mob mistakes him for the beast and kills him. Ralph and Piggy’s presence in that circle makes the killing a shared collapse of civilization, rather than an act committed only by Jack’s tribe.