In a usual northern-hemisphere winter, a few of these so-called major stratospheric warmings take place along with several minor events. That major stratospheric warmings only occur in the Northern hemisphere is because orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2) Rossby waves in the troposphere. These waves travel upward to the stratosphere and are dissipated there, producing the warming by decelerating the mean flow.
There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation: If the QBO is in its easterly phase, the atmospheric waveguide is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex, intesifying their interaction with the mean flow. Thus, there exits a statistically significant imbalance between the frequeny of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).