After watching it VERY CLOSELY , you can see what looks like a shadow going across the top corner of WTC 2 before it appears to come out behind WTC 1. And the trajectory is not in a continuous line throughout the "flight" of the object. I would hazard a guess and say it was a bird flying close to the photographer. This would explain why it's out of focus to the rest of the video, the incredible speed it seems to be traveling and, why it's not traveling in a straight line throught the flight. And why there was no evidence of it on other angles.
Just watch it again, but this time watch WTC 1 right after the plane protrudes out the north side of the building and you will see what I'm talking about.