One record breaking year is hardly suprising. Ozone levels have quite a bit of variance year to year, but the downward trend is now stabalizing and it's expected the trend will be upwards soon. Because of the variance it means it's very easy for a freak year to give record levels.
ftp://jwocky.gsfc.nasa.gov/pub/eptoms/images/qcplots/zmqcsz_v8.png
