Why do the tribunes chase the commoners away, and for what does Marullus reprimand them in William Shakespeare's Julius Caesar?
This is a good question. Let me set the stage for you. We see the tribune of the plebs, Flavius and Murellus in the streets with a bunch of common citizens. It is before the triumph of Caesar.
These tribunes basically tell people to stop loitering and get back to work. When a cobbler states that he is taking a break from work to see the military triumph of Caesar (one of the greatest glories a Roman commander can achieve), Murullus states that the triumph of Caesar is nothing great, because no foreign enemy was vanquished. If you know Roman history, Caesar defeated Pompey, another Roman general in a civil war.
Here is a quote that shows this:
"What conquest brings he home? / What tributaries follow him [Caesar] to Rome / To grace in captive bonds his chariot wheels?"
In a word, Murullus is making little of this military work of Caesar. In light of all this, we can say that the tribunes are pushing the commoners away and even reprimanding them, because they do not like Caesar and what he stands for, the potential end of the Republic.
Shakespeare was both a writer and director of his plays. He had a financial interest in the theater where they were performed. He knew that he was going to need a group of extras to represent the mob that listens to Antony's funeral oration because that scene is essential to the play. No doubt he decided to make additional use of all these men in order to get the most for his money. This could have been his reason for writing the opening scene in which the tribunes chase the commoners away. Then to make still further use of these extras, he may have decided to write scene 3 of Act III, in which the mob murders Cinna the poet. Some of the same extras were probably assigned to play soldiers in the battle scenes at Philippi in Act V.
It seems likely that the young female impersonator who played Portia in the original productions also played Calpurnia and may have played the boy Lucius in Act IV,2.