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. A small insect known as the pecan nut casebearer is the cause of pecan shedding at three different periods of the year; mid-May, July and on rare occasions in late August. 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. Any stress received by the tree at this stage can result in major fruit drop. Some trees can lose up to one half of its crop if not properly managed during water stage. My guess is that even though you have been watering your grass during hot, dry periods, you have not deep watered your pecan trees. Watering grass is not enough for watering trees - slowly soak the tree's root system at the dripline of the tree and not at the trunk.

I hope this answers your question.