Rioting (per Martin Luther King) is the language of the unheard. In general it does seem to be the case that riots start when a large group of people congregate, and at some point there is a spark of dissatisfaction that they do not feel is being addressed by the authorities. Rioting of course, can than often turn to looting:
Social Scientists distinguish different types of looting, including:
• Looting of goods needed for survival
• Opportunistic theft of good such as TV sets
• Collective action, conditioned by the political environment
An exceptional video showing the first two activities can be found here:
Chili Looter Video
You will also note that organized neighbors were the first effective early response.