That is not an explosion (caused by explosives)*. You see that is the problem with just looking at still images. They can be decieving. If you look at the video you would have noticed that the trails appear to arc upward and out from the center because the debris collapsing in the core area sucked the air down toward the center. Look at the videos.
http://www.youtube.com/watch?v=5rsXvK7-l9E
In this video the dust trail looks like it is actuall rolling back toward the center.
http://www.youtube.com/watch?v=HFtzTQKRspY
http://www.youtube.com/watch?v=qRakw3hwPls
http://www.youtube.com/watch?v=oCSXco-bPNo
In each of those videos notice how all the smoke, dust and debris in the center of the building actually gets sucked down in toward the center thus giving the appearance of debris being shot out upward and out from the center.
Do you see now the problem with just interpreting still images?
* I added this because not all explosions are caused by explosives