So it is quite tempting to assume that:
1) SSW happens when the regular atmospheric circulation gets "overridden" by the auroral current systems;
2) [therefore] SSW in the north are more frequent - since the auroral currents are stronger there;
3) the recent increase in frequency of southern SSW is caused by long-term weakening of the solar activity - and therefore higher cosmic ray flux, higher ionization and stronger currents.