I've seen a mini-documentary breaking down the videos from that night. It appeared to me that KR was "stationed" at a certain business in the moments leading up to the first shooting. There was shouting or something that indicated that protesters might be vandalizing a used car lot down the street and KR started running to that scene, apparently to do something about it, whatever that was. Rosenbaum saw KR running and that's when he started chasing him. My personal interpretation of what was going through Rosenbaum's head is, "Here goes one of these motherfuckers!" Rosenbaum was already agitated and seeing one of these armed guys take off running was the switch that got flipped in his head to "do something."
Causation is a series of concentric circles. If the cops wouldn't have shot Jacob Blake none of this would have happened. If KR hadn't shown up in Kenosha this wouldn't have happened, etc. etc. etc. ad infinitum. But Rosenbaum was mentally unstable and I think he is the biggest reason this happened. Add a mentally unstable person to an already tense situation along with people like KR, with no training or experience, and it's a powder keg. It's exactly the reason why, as a policy point, it should be illegal to charge into a protest - of any kind - with weapons and assertions of open carry. It just doesn't work and is going to end in a lot of avoidable deaths.