He chased him because he was carrying a gun and was seen as a threat.Just that it wasn't Rittenhouse who engaged first. Instead he tried to flee, just like Arbery. Prosecutor in the Arbery case stated that Arbery tried to flee, indicating he did not want to engage or have anything to do with the defendants. Same thing with Rittenhouse. Rosenbaum had no business/right to chase and attack him.
Rittenhouse's defense lawyer Mark Richards: