For me the key point is that the power of the sun can let in harmful radiation...

Ozone in Earth's stratosphere protects the planet against harmful ultraviolet radiation. Most of the gas lies in the lower- and mid-stratosphere, where observations have shown a thinning above the poles caused mainly by man-made chemicals, such as chlorofluorocarbons (CFCs). Electron funnel The domino-like effect began in October and November 2003, when the Sun unleashed a record barrage of radiation and charged particles toward Earth. The planet's magnetic field funnelled some of the storm's electrons into the upper atmosphere above the poles. The electrons hit nitrogen molecules there, breaking some of them into nitrogen ions. Those reactive atoms then combined with nearby oxygen molecules to form molecules of nitrogen oxide - levels of which rose in November and December 2003, according to the satellite data. Finally, downward-blowing winds in a polar vortex above the Arctic pushed these molecules into the stratosphere. There, each nitrogen oxide molecule could rip apart hundreds of ozone molecules, just as CFCs do. The effect remained even into July 2004, according to Randall's observations.

Source: New Scientist