I'm not sure justified is the correct word for this question. I think the ease of persuasion amongst the crowd can be easily traced as far back as the start of the play. Flavious and Marullus yell at the commoners for rejoicing and celebrating in the streets at the return of Ceasar although it wasn't long ago they did the same for Pompey. Brutus certainly learned his lesson about allowing a competitor speak to the crowd last. Every time a stronger force make a suggestion to this crowd, they immediately and blindly follow the new speaker.