Excellent observation, since muslims are extremely anniversary aware. September 11 was not picked out of the air.
September 11 is also the beginning of the Ecclesiastical (Church) Year on the Coptic calendar.
http://www.missionstclare.com/english/people/sep11o.html
Just a cursory read of the Orthodox Matins for that day reveals a theme of prayers for peace and victory:
http://www.antiochianladiocese.org/files/service_texts/other_commem/indiction/Sept-01-ORTHROS.pdf
(Start reading on page 6)
September 11 was NOT random.
The jihadists were attempting to poke a finger in the eye of Almighty God!