You just said Rittenhouse initiated the confrontation.
Anyway, he didn't throw the bag until after he had been chasing Rittenhouse.
Kyle put out the fire, Rosenbaum got pissed and made to go after Rittenhouse. Rittenhouse ran and Rosenbaum chased him. At some point during the chase he threw the bag at Kyle. Kyle continued to run (with multiple people chasing him, by the way) and eventually found himself cornered in a parking lot and turned to face Rosenbaum. Rosenbaum tried to grab Kyle's rifle and that's when Kyle fired to defend himself.
Rosenbaum initiated the confrontation. This is proven by the video.