The only cause of that historic disaster (whatever it means) was nothing, absolutely nothing but the utterly corrupt and incapable-of-being-reformed regime of Shah's one-man-show... Short, but understandably not that simple... 

Had shah of Iran put capable and kick ass leaders in charge in 1978 and even in 1979 instead of all the weak and incomptenet leaders he shuffeled through till Bakhtiar came to power (Too little , too late)  this angholab would have not succeeded.

On the other hand with so much pressure by U.S and Europe for shah to depart, its highly  possible that if he stayed, they would have provided enough moral and logistical support to his opponents  to turn Iran into another war zone like Syria, Yemmen and so on.  Listen to Sazgara interview: The  purpose of Rev Guards were to start a deadly  fight with the army (Which would have resulted in chaos, bloodbath and civil war) but the rev succeed fast and with out it!