  Most, but not all, liquid crystals tend to align when subject to shear flow,
while most nematic polymeric liquid crystals undergo a tumbling instability,
where the director rotate with the flow. The reasons of this instability remain
elusive, as it is possible to find similar molecules exhibiting opposite
behaviors. We propose a theory suitable for describing a wide range of material
behaviors, ranging form nematic elastomers to nematic polymers and nematic
liquid crystals, where the physical origins of tumbling emerge clearly. There
are two possible ways to relax the internal stress in a nematic material. The
first is the reorganization of the polymer network, the second is the alignment
of the network natural axis with respect to the principal direction of the
effective strain. Tumbling occurs whenever the second mechanism is less
efficient than the first and this is measured by a single material parameter
$\xi$. Furthermore, we provide a justification of the experimental fact that at
high temperatures, in an isotropic phase, only flow alignment is observed and
no tumbling is possible, even in polymers.
