In Julius Caesar, Act I, why are the workers celebrating at the end of Scene 1, and why does Marullus scold them?

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.