A Tunisian offers a useful perspective at the Aqoul blog:
The way riots spread from rural Tunisia to urban Tunisia was through rural exodus and timing coincidence. The details about what occured when exactly vary according to versions - but they generally involve repression during a market day. The most likely version is that Bouazizi's immolation, or his relatives' angry reaction against the administration, would have happened then.
That is, when his relatives started throwing rocks at the police station and/or the governorate and/or the town hall buildings, police reacted with their usual violence, and that further angered the people in the market. Snowball effect to nearby towns and then to the usual unrestive interior and southern Tunisia. Center-coastal (the usual source of the ruling and economic elite) and North (center of power with Tunis) joined only later, when the relatives of interior and southern Tunisians who moved to the capital for economic reasons reacted. They started rioting against the police because of the repression going on in their villages, building up from a few incidents in some working class neighbourhoods of Tunis to a widespread movement.
Human rights and democracy, which two months ago generated cynical smirks and were synonymous with Western oppression of Arabs, suddenly became something to strive for and regained their semantic meaning. The merit goes to laywers and other civil right activists who were the first movers after "the street" and who were key to mobilizing the middle-class and other organizations later.
Unions, which appeared as the primary force after the fall of the regime, were initially split on whether to support it or not. They really united against him late in the stage, and there are pending accounts to settle as a result. The current leadership is contested and its time is probably counted as a result. The first participation was driven by grass-root unionists - basically part of the street - who have only been able to impose their view once it was clear enough that the balance of power changed against Ben Ali.