Here is an article by Waleed Ziad, in the New York Times, titled Jihad's Fresh Face. In it, he traces back violent militancy, in which current terrorist movements are grounded. The root cause is post-colonial chaos created in the Arab world that lead to the rise of absolute dictatorship. When these dictatorships are challenged, they ruthlessly and brutally suppress the opposition. This suppression did not stop at banning and imprisonement or those who opposed, but more heinous measures such as torture, continued persecution, harassment, ...etc.

All this led to the rise in the 1960s of the ideology of Sayyed Qutb, which was later adopted by several groups. This ideology is rooted into revolutionary radicalism, aiming at overthrowing the dictatorships, but wrapped in Islamic terminology.

From this ideology, Ayman El Zawahri and his likes emerged, who later merged his group with Bin Laden.

An the rest, as they say, is history ...