In a recent Letter, we showed the remarkable result that the atomic alignment of the levels P1/2 and S1/2 of the D1 line of Na I is practically destroyed in the presence of magnetic fields sensibly larger than 10 G, irrespective of the field direction. In this paper, we analytically demonstrate that this property is a consequence of the decoupling of the electronic and nuclear angular momenta J and I in the excited state P3/2, which is achieved when the Zeeman splitting from the local magnetic field becomes much larger than the typical hyperfine separation for that level.