Aft C of G reduces directional stability. Dutch Roll is worsened when this occurs as the tail restoring moment is reduced. Dutch roll is significant when directional stability is low compared to lateral stability.
At higher weights in the 707-320B Dutch roll is worsened as the Moment of inertia about the yaw axis is also increased. In the case of the centre tank on the 707 the Moment of inertia effect is more than the Forward C of G effect. Also the Centre of lift is moved forward at High AoA(due weight) and so the drag effects due to subsequent yaw are worsened.
