Their beef is the inequality.
Whites get to throw haymakers, run, shoot, kill cops, etc and get to be paraded into the booking station maybe with a little scuffle at worse. Perhaps even a complimentary Burger King meal if they shoot up a black church but I digress. Look at the last few cop killers: all white and nearly all of them were taken in alive. That's a fact.
In that area of Milwaukee, it's been building up for a long time, according to reports. Very segregated by way of bridges and rivers. The BP gas station that was burned down--the owner had ran out shooting at the locals and cops ignored it, so the burn down was retribution. The locals do not believe the police reports about what happened with the shooter.
If the body cam footage doesn't match the mayors story of the suspect carrying a gun in is hand and all this and that, there will probably be more trouble.