The ozone hole is caused by the effect of pollutants in the atmosphere destroying stratospheric ozone. During the Antarctic winter, something special happens to the Antarctic weather.
Firstly, strong circular winds form that blow around the whole continent, this is known as the "polar vortex" - this isolates the air over Antarctica from the rest of the world.
Secondly, special clouds form called polar stratospheric clouds. Clouds don't normally form in the stratosphere and these turn out to have the effect of concentrating the pollutants that break down the ozone, so speeding the process up.