It's quite complex but it boils down to lack of opportunity and with it a lack of decent jobs and the ensuing poverty. I live in the North of England and there's been rioting near me.
Lots of towns in the North and Midlands grew up around certain industries. And when those industries closed the heart was ripped out of them.
People living in these areas feel left behind, ignored, undervalued. Whether they are or not (and fwiw I think many places are) is immaterial - that's the perception.
And when you feel like that, you want someone to blame. Because why are so many people doing alright when you and yours are having a shit time?
In come the far right - the experts at placing that blame on people who are brown and black, immigrants, people who aren't Christian etc. The far right has long attracted some highly competent communicators and calculating people: Oswald Moseley, H*tler, Farage etc. They excelled at getting messages across.
And if you live in a town with a lack of opportunity and you see hotels housing asylum seekers or people of colour committing a crime while you're struggling then that's your "proof" that what articulate, intelligent, bloke from the BNP said is true. And finally someone's "listening" to you.
And that's how it happens. That's how the far right get a grasp in areas like Rotherham. People in Rotherham (to give one example) aren't stupid or naive - they're fed up of their lot. And their blame goes somewhere. And today it's on Muslims and asylum seekers. In the 30s, it was on Jews.
Oh and then some thugs just want a riot. Doesn't matter when, where or what. They just like rioting and here's an excuse.
And no, I'm about a million miles from being far right myself. I'm the polar opposite. I loathe the riots. They're horrific. But it's surprising - given what a divided society we live in - that we don't see more of this.