Local news reporting tries to explain what's going on: https://www.youtube.com/watch?v=SJGT06zIUiY&t=2m01s
"What you've been seeing is basically a cat and mouse game all night. The police have been here with a huge show of force, but they can't stop people, because they don't know that they've necessarily been looting. And so they're waiting for somebody to do something, police move onto the next block, and then someone does something."
I don't know one way or another whether this is the whole truth, but it appears to play a significant role according to the reporter.