Wednesday, 5th January 2011, 16:26 by jpeg
If you are not wearing a shield at the time of acquirement, getting a shield becomes more likely. I don't know the exact chances, but if all other slots (i.e. helmet, gloves etc.) are filled or unavailable for your current species, but you don't have a shield yet (or at least, aren't wearing one) the chances of getting a (plain, non-randart) shield are pretty good.
In trunk, this has been changed to have acquirement look at armour types seen so far rather than worn, so if you really don't want to wear a shield, that becomes less of a nuisance.
Otherwise, you might simply be experiencing a shield-happy string of random choices.