I am not aware that the answer to that question is even known. Possibilities include atmospheric circulation and anomalously large amounts of cosmic rays entering the polar regions, funneled there by the Earth's magnetic field.

The hole in the ozone layer is centered over Antarctica. The reason the hole doesn't move is due to several factors. One is that there is an Antarctic vortex. During the very cold winter temperatures, the slow rotation of air around the South Pole forms a partially walled-off mass of air the size of North America. Although there is also an Arctic vortex, it is weak by comparison. Antarctica is also unique in that it has polar stratospheric ice clouds, or PSC's.

In the October Spring when the sun rises, rapid reactions on the surface of the PSC's cause the equally rapid destruction of ozone molecules. The Antarctic vortex slows the mixing of air with the rest of the globe, and thus we get a fairly defined ozone "hole" over Antarctica. Hope this helps.

Karla Wilson, MadSci Network at link