As I have understood it, it is due to the rotation of the Earth.
If one is stationary at the equator, hten one is moving at some speed which is the rotational speed of the Earth. If then one moves directly to the North, then the rotational speed of the Earth is the decreses but the angular speed is the same, due to smaller radius of the circle. Therefore one is moving at the same rotational speed as eqator, which is higher than at this northern lattitude, and it comes out as a turning force. The same happens when going from north to the Equator, one's speed is less then the Earths at the lower lattitudes, which comes out as a turning force.
Please correct me if this is far off
