Why are the people of Rome celebrating in the streets?

At the beginning of Julius Caesar, Flavius encounters a pair of commoners celebrating in the streets and questions what they are doing, stating that it is not a holiday and demanding to know the men's trade. The men resist but eventually admit their trades, and Flavius presses to know why they aren't in their shops and at work but are instead wandering about in celebration.
The second commoner jokes that he is leading men around to wear out their shoes so that he, a shoemaker, will have more work, but then he explains that they are celebrating in order to see Caesar "and rejoice in his triumph."
Marullus thinks this is ridiculous, pointing out that the people of Rome had once loved Pompey, too, and now they came to "cull out a holiday" for Caesar, who "comes in triumph over Pompey's blood." He is suggesting that the people are fickle in their affections.
He does, however, point out that "it is the feast of Lupercal," so there is some underlying pretext for the celebration, but it has come to be centered very much on Caesar. Flavius is keen to ensure "no images / Be hung with Caesar's trophies." The implication is that the people are hailing Caesar as if he were a king.