The J-curve theory of social movements. But it goes only as far as explaining why people join ANY movement challenging the status quo (i.e. the "push-factor"). They do not explain, however, why people join a PARTICULAR movement (i.e. the pull-factor). Why did Latin American students were attracted to radical Marxism, while Arab students to radical Islamism? Or for that matter, why are the frustrated US students attracted to anything from Christo-fascism to rap and to anarchism?
I think that the resource mobilization approach, which concentrates on the supply-side of social movements (i.e. agents whose goal is to mobilize a particular constituency) does a better job. Islamism became popular because it was popularized by mobilization agents - e.g. Islamic clergy with access to Saudi money and Western communication resources.
Wojtek