When a quarterback throws a perfect spiral deep ball, the ball is initially angled upwards, i.e. the front tip of the ball is higher than the back tip. In the air, the ball levels off at the peak of its flight with both tips at the same height. As it begins its descent, the front tip begins to fall lower than the back tip and it eventually lands in the receivers hands at about the same, albeit reverse, angle as when it started its journey.
Why? Why doesn’t the ball just maintain the original angle all the way through?
The short answer is that the spin on the ball generates an angular momentum vector that is roughly parallel to the velocity. The spin also generates different amounts of aerodynamic drag on the left and right sides on the ball; you can read why here and here (PDF).
The uneven drag pushes sideways on the ball’s center of pressure (which leads the center of mass), and this torque causes a net downward force (right hand rule!) on the front half of the football, pushing its nose down.