There are times when nut bearing trees like hickory, walnut and pecan normally drop some fruit well before full maturity. There is always a natural shedding of a portion of the annual nut crop and should be expected. There are other causes that can be more problematic, including adverse weather conditions, tree health, poor pollination, insects and disease. Find out more about these "abnormal" causes of the dropping of a season's nut.
Walnut Anthracnose - FIDL/USFS