So does Bristol; they seem to happen once a decade or so :)
Usually it is helped along by the feral youths from one deprived area "visiting" another deprived area on the other side of the city.
Last time it was a little different: the hippies in the "telepathic heights" squat were abetted by more respectable people protesting against a new (rapacious) Tesco supermarket opening up in "the longest row of independent shops in Europe".
In either case the local constabulary is traditionally helped out by the boyos in their vans marked "Heddlu".