The construction of the twin towers began in 1968 (same year that 2001 A Space Odyssey came out).
1968 is 33 years before 2001. 33 is a special number in Freemasonry. It is their highest degree.
The numbers 9 and 11 also are special numbers in the occult. Aliester Crowley said that 9 and 11 are the most Satanic numbers.