When I'm scouting for pecan nut casebearer, I always notice some level of post-pollination nut drop. The other day, I was able to photograph a single nut cluster that exemplifies two common causes of post pollination nut drop.
The olive colored nut at the top of the peduncle is an example of a second type of nut drop. This pistillate flower was most likely pollinated but the egg inside the ovary was never fertilized. Lack of fertilization can be the result of improper pollen tube growth or a genetic miscue between egg and sperm. In the photo you can also seeing the beginnings of an abscission zone forming on the peduncle right above the healthy nut (small crack). In shedding the unfertilized nut, the tree will also shed the upper part of the peduncle above the abscission zone.
This year we are experiencing an "on" year for pecans. The vast majority of our pistillate flowers were healthy and are now developing into a bumper crop of pecans. In contrast, during "off" years we seem to see a lot more post-pollination nut drop of both types illustrated by the photo above.