One possible explanation for this asymmetry is large-scale convection above the core. The convection can create variations in the local abundances of elements, resulting in uneven nuclear burning during the collapse, bounce and resulting expansion. Another possible explanation is that accretion of gas onto the central neutron star can create a disk that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova. (A similar model is used for explaining long gamma-ray bursts.) The dominant mechanism may depend upon the mass of the progenitor star.