There is, of course, no evidence that the people shot were rioters. Not that it's going to stop certain people from pretending that they were.
Rosenbaum was homeless after having been released from the hospital after a suicide attempt, and based on his reported behaviour he was having a mental health crisis at the time.
Huber, a friend of Jacob Blake, had gone to the scene of the unrest to document the demonstrations for posterity according to his girlfriend Gittings.
Grosskreutz was supposedly there as a medic to help out with medical care.
As to why they were assaulting Rittenhouse, it's hard to know what set Rosenbaum off. It may have been that Rittenhouse was armed (He was at one point asking armed people to shoot him), it may have been something else.
Huber most likely thought he was seeing someone who'd just shot and killed someone, and tried to stop the shooter from getting away or killing others. His girlfriend suggests he was trying to protect her.
Grosskreutz... you'd have to ask him.