Order of magnitude estimates suggest that optically controlled bulk semiconductor switches should be able to withstand voltages up to the product of their thickness and the dielectric strength of their material. In reality, however, the devices fail--i.e., exhibit a behavior that resembles dielectric breakdown--already at voltages which are much lower. This deficiency threatens to limit the prospects of the device concept quite seriously and has so far not completely been understood. In our paper, we discuss several mechanisms which may underlie the observed phenomenon, and focus in particular on the dynamical aspects of it, namely on the sudden transition ('sudden breakdown') which takes the switch within a few ns from the resistive off-state to a highly conductive on-state. We investigate a scenario that relates this transition to a second effect also seen during breakdown, namely to the spontaneous onset of current filamentation, and speculate that the magnetic self-contraction of the current (known as the 'pinch effect') may play an essential role in the process. On the basis of a mathematical device model which incorporates the effects of particle transport and magnetic interaction, we obtain quantitative results for the speed and the threshold of magnetically driven filamentation, and find those numbers to lie in the A and the microsecond(s) region, respectively. We conclude that the magnetic pinch may play a essential role in the dynamics of current filamentation and fast breakdown, but cannot explain the fast observed current rise in the ns-range by itself.