What instigates the fight between Abraham and Balthasar (Montagues) and Sampson & Gregory (Capulets)?

The fight starts when Sampson and Gregory (Capulets) decidedly anger Abram and Balthasar (Montagues) in order to get them to pick a fight. Sampson bites his thumb at them and this angers Abram. Sampson makes fun of Abram for being angry yet refusing to fight, and the four of them duke it out.