The BBC take on this conforms with my understanding, which is that the far-right nationalist thugs have been both emboldened by Brexit, and infuriated by the fact that Brexit has completely failed to result in a massive reduction in the number of people with black and brown skin living in the UK.
Of course, you would have to be a total fucking idiot to think that leaving a European Economic and Political administration would lead to the departure from the UK of UK citizens who had grandparents or great-grandparents from the Carribean or the Indian subcontinent.
The two concepts are completely unrelated, other than in the pathetic "reasoning" of white supremacist morons to whom the world is divided into "Real English People" and "Foreigners", with the latter being an undifferentiated mass of evil, poisoning the efforts of the former to restore a world that never existed, wherein being a white, English, football-loving skinhead was an automatic ticket to success and respect.
The putative "cause" of this violence, attacks by Muslim boat-people against English children, are fictional; But the rioters neither know nor care. They get their "information", along with their coordination, from rightwing websites that make the Daily Mail look like paragons of tolerance and kindness.
These thugs may wear England football shirts, rather than brown ones; But their tactics and aims would be completely unsurprising to the leaders of the SA in the 1920s and early '30s.