Why is Flavius upset with the commoners' behavior at the beginning of the scene?
Flavius and Marullus are out removing the decorations from the statues. These decorations have been placed there by people who are welcoming Caesar home as a hero. Instead of being at work, most of the population seems to be having a holiday, a fact that greatly distresses Flavius and Marullus. The problem Flavius and Marullus have is that, not so long ago, the crowd was reacting the same way to Pompey, the man Caesar has defeated in battle. Marullus says:
“O you hard hearts, you cruel men of Rome,
Knew you not Pompey? Many a time and oft
Have you climb'd up to walls and battlements,
To towers and windows, yea, to chimney-tops,
Your infants in your arms, and there have sat
The livelong day, with patient expectation,
To see great Pompey pass the streets of Rome”
In other words, he is calling the people who are out in the street fickle. They cheered and were “loyal” to Pompey, but now that Caesar has defeated him, they no longer care about him, and they have switched their allegiance to Caesar.