I think I'd have to agree with Hydor on this one. Seems more likely that there was an imperfection in the glass and you had the bad luck to put the magnet right on it. See if you can find a glazier willing to look at it. I had a window in my apartment crack and the glass maker was able to tell immediately from the nature of the crack that it was due to the building shifting and putting weight on one corner. Maybe one could pinpoint the cause of the crack for you.