Why are the tribunes Marullus and Flavius angry with the plebeians?
The tribunes Marullus and Flavius are not only angry with the plebeians because they are cheering for Caesar. Yes, Marullus and Flavius no longer respect Caesar, fearing that Caesar's ambitions no longer reflect what is best for Rome. But what bothers the tribunes more is that the plebeians are so fickle. The tribunes point out that the crowd of workers, not so long ago, were equally cheering for Pompey, whom Caesar has now killed.
Note that this opening scene is reflective of another scene that includes masses of the common folk, a scene that also demonstrates the fickleness of the crowds. This other scene occurs after Caesar has been murdered. At first, Brutus is able to sway the crowd to support the assasinators. But as soon as Antony speaks to the plebeians, they are convinced that the assasinators are wrong. So the opening scene of this play acts as a foreshadowing of the climatic portion of this drama.
The crowd of Plebians are celebrating Caesar's victory. Marullus and Flavius are opposed to the celebration because they are concerned for the state of Rome--they are worried about the power Casear has acquired, adn the effects that it may have on Rome.
Marullus and Flavius are Tribunes men. They are angry so they want to drive the commoners from the street because they didn’t like how they were all praising Caesar. Marullus and Flavius take down anything that is praising Caesar to further hinder the celebration of Caesar’s victory.