Short version of this: African negroes are living (illegally) in a building that was part of the former Olympic village. The incident may have started with a dispute in a bar between some of the "refugees" and a group of Turin football fans. It seems the tifosi were aggrieved about the conduct of the negroes and wanted to teach them a lesson. So they went to the negro building and one or more explosions was heard, possibly caused by firecrackers, although some accounts mention letter bombs.
The negroes then go crazy, run out of the building and start rampaging in the street, overturning objects, throwing stones and bottles, while shouting "Bastards!", "Racists!" and "Swine!". This goes on till about midnight. Mass police forces wearing riot gear turn up to contain Turin's first "Migrant Revolt".