Why did this cop smash this woman in the face during the Arizona "riot"?

Well away from the "front line" of this weekend's sports-related disturbance in Arizona, a cop dashes up to what appears to be a calm, unthreatening spectator and strikes her in the face (0:20s). She's thrown over a bench; the officer makes no attempt to detain, arrest, or otherwise communicate with her.

The officer just turns and walks away.

Answers on a postcard, please! [Video Link]