If you didn't hit a pothole, haven't had a puncture causing the bike to run on the rim, haven't exceeded the max. pressure for the rim, then I'd suspect a manufacturing fault.
If the wheel was fairly new you'd be able to argue for refund/replacement, but as it sounds as if the rim's older than that you could try a letter or email with photos to the makers.
Worth a search on forums etc. first (here, CTC, YACF and others) to see if other people have had the same thing with the same rims.