That's a good one. You can see the blasts going off on the side. That's why the top section "righted" itself, because the opposite side of the building was being blown. If those explosions hadn't occurred, the top section would have kept following its downward angular momentum. But it didn't.
Some interesting things on this forum. I forgot about this:
Another report came from Scott Forbes, an employee of Fiduciary Trust, a firm which was located on floors 90 and 94-97 of the South Tower. Eighty-seven employees of Fiduciary Trust were killed on 9/11. In an email account, Forbes reported that over the weekend of September 8-9, 2001, floors 50 and above of the South Tower experienced a "power down," meaning that all electrical current was cut off for about 36 hours. The reason officially cited was that the electrical cables in the building were being upgraded. Forbes was an information technology officer in charge of Fiduciary Trust's computer network; his attention was engaged by the power down because it fell to him to shut down all the company's computers and related systems before the power went out. After the power down, he had to turn the computers back on again, and restore service on the network. Because there was no electric power above the fiftieth floor, there were also no security cameras and no security locks. There were however many outside engineering personnel coming in and out of the tower at all hours during the weekend. Forbes lived in Jersey City and could see the WTC towers from his home; when he saw the conflagration on the morning of 9/11, he immediately related it to the events of the previous weekend." (Tarpley)