Look at some examples of decorative calligraphy. They can make birds and all sorts of stuff out of letters. To make Twin Towers out of letters would not be a problem. Look at the letters. Other designs are possible. One alternative design would be to emphasize large, circular loops. Instead the calligrapher highlighted extended, dual vertical strokes, making Two Towers out of them with debris falling from out.
In the Arabic alphabet, even in highly decorative calligraphy (which this is not--it's pretty close to how Arabic is printed in most books), there are rules that need to be followed. To write mubaarak you need to write an extended aa, which is done be extending an alif (a) out of the preceeding consonant. As the alif is what is called a disjointed letter (not joined with the letter that follows), 'baa' looks like a backwards J. The other 'tower' is the Arabic 'k'. Like 'k', it is written with a tall portion, so it looks similar to a J with a flat bottom. In the case of the word 'mubaarak', k is preceded by an Arabic 'r' (its the little squiggle between and slightly below your 'two towers'). The Arabic 'r' is what is also a disjointed letter, so the r and the k are not connected, which is why the tall part of k stands alone.
The 'extended, dual vertical strokes' are neccessary for the word mubaarak to be written.
Either the calligrapher was only following the standard rules of the Arabic alphabet, or he was a member of a vast Muslim conspiracy and had full knowledge of 9/11 months (if not years--I don't know when the calligraphy was done) before it happened. I know which choice is the more rational one.