There are numerous causes of premature pecan drop. Some varieties such as 'Desirable' shed naturally. Poor pollination results in a drop from June through July. Planting several varieties helps reduce the poor pollination drop problem. This drop is easy to identify because there is a small hole in the base of the pecan.
Water stress can also result in pecan drop.
Ideally pecans should be watered every two weeks. Three weeks without water is the maximum. Nutritional problems from shallow soil or poor fertilization can cause pecans to shed throughout the year. Water stage in later July and early August is the most common form of pecan drop. As the nuts move from size development into kernel formation the pecan sheds very easily.