Since the fins are rotationally symmetric about the long axis, net force generated by fins cannot introduce a moment about the tipping axis, since it would mean a net force perpendicular to the axis of symmetry (which cannot happen, by degrees of freedom only a free moment along the axis of symmetry and a force parallel to axis of symmetry can exist) . It can be rather concluded that tipping is caused by the location of aerodynamic center and center of gravity made falling parallel to the long axis is the most energetically stable position to fall in. It is however true that in conventional airplanes, the incoming horizontal airflow is redirected downwards to generate lift by conservation of momentum. Your mistake here was equating fins to airplanes on wings. On airplanes, the angle of attack are the same relative to one reference frame. With rotationally symmetric wings around a rounded body, angle of attack is only same relative to the rotating reference frame.