Hey, bullthistle is right. Willows like plenty of moisture and if yours was used to "perfect" watering in the nursery its a natural defense against drought to shed leaves. But, if you haven't weilded the pruners yet - scratch the defoliated stems with your thumbnail to see if the twigs are green under the bark. If so, you might wait and see how far out they leaf out again and prune it then. If brown, chop chop.