There is an element in which this is a vicious circle, really. Historically people from different races have been treated with contempt, and while this is less so now than in the past it still looms large in some people's heads -- it's clear that more than a few people have chips on their shoulder. And then the vicious circle bit starts where, say, an action can be claimed to be racially motivated (even if it isn't) and then this gives people the excuse to behave badly; which in turn feeds suspicions that people of a different racial background can't be trusted, which might lead to more such incidents with racist overtones; until it barely takes a whisper to set people off either in crying "Racist!" or in wondering if this is an actual trend.
The reasons in reality are far more complicated than the colour of skin and could include upbringing or perhaps incidents in any particular person's past, and so on. Upbringing in particular is important -- while it isn't anything to do with racism itself, how much clearer can you be that what is said at home is important than when witnessing all those people who weren't even alive in 1990 celebrating the death of Thatcher as if she had wronged them personally? Or in the apparent cycle we sometimes see in Northern Ireland where the old generation tires of violence itself but manages to enrage the youth so that the cycle starts all over again?
Riots of this nature expose deep, deep issues within and between communities, how they are perceived by others, how they feel they are perceived, and how they react. Leave it long enough and the initial reasons for the distrust and hatred are forgotten, and there is only the hate and violence.