Well Tajfel's Social Identity Theory is a seminal piece of psychology. Before Tajfel, people thought that conflict arose because we have a basic irrational, aggressive urge to protect our group: the 'blood and guts' model.
Tajfel however showed that group prejudice - to prejudge - was an entirely rational response to social context.
That society is relational so your perceived social group will inevitably be compared to another and from that conflict will arise as we are programmed to process and categorise objects in our brain in a way that protects our self esteem and therefore the status of our group. He showed this through experimentation which has been replicated many times.
Therefore we can't help bias, it is programmed into us and therefore conflict is, to some extent inevitable, and perhaps not necessarily a bad thing (apart from all the death and destruction)
So in the context of these riots - it seems there were many groups in conflict but broadly there seemed to be the impoverished young who identified themselves as a 'group' against everyone else. They want to maintain or raise the status of their group as it feeds directly into their individual self esteem.
Therefore they asserted themselves to show power against the police and against us. Again the looting of the shops was about simultaneously acquiring goods that provide status (flat screen TVs/trainers/phones) and destroying them (fuck you, you can';t have them, I don't care about your society anyway.)
WAKE UP AT THE BACK
But Tajfel's assumption that an individual's motivation will be to protect 'self esteem' and his discounting of our innate irrationality has been criticised More work has to be done on the role of 'emotion' and the unconscious in provoking extreme behaviour. (Although Tajfel was not a fan of this approach to psychology.)
Frankly I like my 'some people are just bastards' analysis but I doubt it would get me many marks in my essay. I am sure there some psychologists on Mumsnet who can explain this far better than me - I am just an OU undergraduate 