We present an analysis of the formation and evolution of the transient periodic pattern in the nematic director field reorientation in the magnetic non-Freedericksz twist geometry and compare with experiments. The stability analysis of the magnetically induced director reorientation in a planar oriented nematic slab with rigid anchoring and with the viscoelastic parameters of 5CB shows that, when the magnetic field is not normal to the undisturbed director no, the transient hydrodynamic instability sets in for a periodic pattern that is oblique with respect to n(0), in agreement with our experimental results for a nematic slab of 5CB. The analysis of the formation of the stripes show the equivalent of a tricritical point separating supercritical from subcritical bifurcations when the critical wavevector at the transition is plotted as a function of the reduced field. The analysis of the evolution of the stripes show that the amplitude of the periodic perturbations only grows significantly near the Freedericksz geometry. This is also in agreement with our experimental results, where the stripes are only observable in the vicinity of this geometry. Our results show that there is a favourable range of the magnetic field intensity for the instability growth which corresponds to a minimum of the obliqueness of the stripes. We also show that the time for reaching the maximum distortion amplitude decreases with increasing magnetic field while for given field it increases when the Freedericksz limiting case is approached.