All Duggans death did was cause a protest march
no one in Tottenham was thinking lets riot, lets loot and steal.
The 'rumour' of the assault of a protester caused a little anger among the towns folk
again though no one was thinking of looting or stealing but some wanted physical and actual retribution against the police so started pelting them with missiles
As this was happening it was being locally reported by BBM messenger and text messages that police were not reacting to provocation so more local undesirable joined in and set fire to vehicles too.
The news cameras came down and were filming (at the time) people burning a couple of cars and a bus and a shop as the police protected their station.
Then it turned into 'look the police are not moving and letting us do what we want'
Then every idiot, criminal, gang member, one parent child, opportunist in the area took that as a sign to loot and steal and burn with impunity, free from consequence as the police were not getting involved and vastly outnumbered.
People saw this on the media and copy catted, it was only when the police started heavy handed, zero tolerance, proactive policing that it stopped.