It has just come out that the reason Rosenbaum started chasing Rittenhouse to begin with was that he had just lit a fire in something on wheels that he was planning on pushing into the police, and Rittenhouse retrieved a fire extinguisher and doused the fire. This enraged Rosenbaum, and he began to chase him, followed by others who were also angered by Rittenhouse having doused the fire.
Where did you get this information?
Here. But I just re-watched it, and it appears there was a short gap in time between Rittenhouse dousing the fire started by Rosenbaum and Rosenbaum and friends chasing Rittenhouse. But doubtless it was this anger that caused Rosenbaum and friends to target Rittenhouse in particular for a beat down.
