"He passed on a perfect shot, choosing instead to allow the motorcade to turn left and proceed below his window, and then took a much more difficult shot with his view partially obscured by a tree."
Maybe he had the shot lined up but hesitated at seeing JFKs face. It was to his advantage that the shots were taken from behind, as all attention / momentum would be rolling away from the scene. With the sun coming in the window it would also be an advantage to hide in the shadows with an eye toward the other sixth floor windows. A straight shot at the motorcade head on would have made for greater exposure; possibly the need to stick the barrel out the window.
Witnesses reported seeing the barrel of a rifle being pulled back into the opening after the shots.