The workers are celebrating the triumph of Caesar over Pompey and Marullus is scolding them because of their hypocritical display since they are the same people who also cheered Pompey a while back.
Caesar, Crassus and Pompey had formed a political alliance in order to dominate Roman politics. On the death of Crassus, Pompey aligned himself with the senate while Caesar went on territorial conquests. Caesar met much success which was seen to obscure Pompey’s own. Pompey and the senate resolved to have Caesar step down from military command which he refused and instead crossed the Rubicon, entering Roman Italy while armed. A Civil war erupted ending up in the death of Pompey and victory for Caesar who went ahead to assume the controls of the Roman government and establish the Roman Empire.
In Act 1, the workers are out in their fine clothes to welcome and celebrate Caesar’s victory and the fall of Pompey much to the disdain of Marullus who was loyal to Pompey.