Why are tribunes Marullus and Flavius angry with the plebeians in Julius Caesar?

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.

In the opening scene of the play, two Roman tribunes named Flavius and Murellus walk through the street criticizing the commoners for celebrating Julius Caesar's recent victory over Pompey. The two tribunes diminish Caesar's accomplishments and resent his growing popularity. After scolding the commoners, Flavius instructs Murellus to go to the Capitol and remove the decorations on the statues honoring Caesar. While Murellus removes the decorations on Caesar's statues, Flavius attempts to force commoners off the streets to diminish Caesar's support and popularity. In act one, scene two, Casca informs Brutus and Cassius that Caesar refused a crown three times in front of the Roman masses before having an epileptic seizure. He then says that Flavius and Murellus were "put to silence" for pulling down the scarves off Caesar's statues. Casca's comment implies that both Flavius and Murellus were executed for disrobing Caesar's statues, and no other information regarding their fate is discussed in the play.

This is a great question. A little context will be necessary. In the beginning of the play, both Flavius and Marullus give the commoners a hard time for wanting to cheer for Julius Caesar. These two men (tribunes) do not like the crowds fickleness. Were they not cheering for Caesar's opponents in the past? Moreover, they do not like the fact that Caesar defeated another Roman in civil war, rather than a foreign army - not to mention they feel threatened by Caesar's power.

As the play progresses, the crowds begin to love Caesar more. There is a statue of Caesar and it is decorated to honor him. During the triumphal parade, Flavius and Marullus take down the decorations from Caesar's statue. When this is reported to Caesar, they are both punished. In particular, they are removed from office for this action.