What causes the conflict between Heathcliff and Edgar in Chapter 7? How is Heathcliff punished?

When the Lintons arrive, Nelly sends Heathcliff out to join the party but Hindley sends him back to the kitchen. Edgar Linton makes a joke about Heathcliff’s hair, causing Heathcliff to grab a pan of hot apple sauce and throw it in Edgar's face. Hindley shuts Heathcliff upstairs, but Catherine tells Nelly to bring Heathcliff down and give him something to eat.