Doing the work of the owner class

It is in the interest of the elite owner class to have police be highly intimidating, which can only happen if they use violence often enough to be a credible deterrent to those who seek a more equitable society. That violence needs to be justified. When protestors are 100 percent nonviolent and are assertive but not aggressive, police violence is not tolerable to the wider public and serious nonviolent activists know this.

I can almost picture the directives issued to the agents provocateurs:

• Riot
• Scream with rage
• Throw things
• Shut down mass transit 
• Break windows
• Accuse anyone who tries to keep it peaceful of being a sellout
• Shove media crews
• Threaten cops

 All these tactics alienate the average person and make them more grateful for cops, more likely to excuse police brutality, and far less likely to feel further sympathy for victims of police violence or for claims that police violence is systemic. All these tactics freeze the development of a movement and ultimately can even roll back advances made when the movement was strictly nonviolent.

Ferguson. Was this done by rioters or agents provocateurs to alienate protesters from general public?

The movement seeking justice for Michael Brown and seeking reform in policing in the US is mostly nonviolent, which is making some advances possible. To the extent the movement features the behaviors listed above, it stays relatively small. Yes, it's national, but it's generally far smaller in each locale than it could be. The movement needs to succeed for the good of us all.