9/11 was chosen as a call to re-establish the caliphate amoungst the Islamic world. As in 9/11/1683. Having spent many years in the ME, I had this explained to me by Turks and others. They all get it.
Sept. 11, 1683 was the Battle of Vienna, the outcome of which stopped Muslim invasion into the West (at the time, represented by Western Europe). The outcome was decisive against the (Turkish) Ottoman empire.
Sounds as if the Muslims are itching for a grudge match. And this time they have chosen Putin as their adversary. Good luck with that. Muslims, on to the glory of the final battle! And please pass the popcorn!! :-)