Riots must be due to basic forces underlying human and social behavior since they have happened throughout history in all parts of the world. Recent years have seen riots not only in Britain, but also in France, and China, to take only a few examples. As Posner indicates, the United States has a long history of riots that date back to the revolutionary period in the 18th century. I will discuss a few ways to reduce the incidence and severity of riots, but the pervasive social behavior that cause riots will always be lurking in the background.
To understand riots it is useful to divide the population of any country or city into three largely distinct groups. Many men and women will basically obey the main laws under the vast majority of circumstances, no matter what others may do. A minority of individuals will engage in criminal behavior, such as theft, if the gains are large enough, and if they are not likely to be substantially punished. The third group comprises individuals who are law-abiding if people around them are law-abiding, but who can be triggered by events and the behavior of others into various acts of lawlessness.
The first group will essentially never participate in riots, while criminals will always participate if they see opportunities to steal and rob. Since riots are social phenomena, the third group of individuals forms the backbone of rioters since they may join in when others are rioting, and refrain from vandalism and stealing if others also refrain. The crucial point in understanding riots is that behavior dependent on what others do has an inherently unstable component. This means that many overall outcomes are possible depending on various precipitating events.