I'm American and don't know much about the situation in London, but I don't think it's a singular event that causes riots. I think it's a build up of tension that escalates opportunistically when something like a police shooting happens.
In this day and age, there is no need for violent protesting in advanced countries. There are so many better, non-violent ways to voice an opinion or belief, especially with today's technological advances. And the saddest part is that the people who are most affected are small business owners who did nothing to deserve harm.
This just supports my cynical theory that most people are inherently selfish and stupid.