A projectile from the South Tower changes direction midair, emitting a white smoke trail.
Gravity doesn’t do that. Nano-thermite does — both the thrust capable of altering the direction of the projectile and the white smoke trail, which is aluminum oxide from the ongoing thermitic reaction. And no, random debris doesn't change direction and emit white smoke trails.
And let's not ignore that ALL of the steel sections are emitting white smoke trails from the thermitic reactions. This one just happens to change direction.