Citrus trees produce far more flowers than they could ever ripen as fruit, so a good percentage of the flowers fall off without making fruit. But, the citrus tree can still set more fruit than it can carry to ripeness. If it does, it will drop some of the fruit during the season to adjust the crop to what it can carry. This is disappointing, but if the tree tries to ripen too many fruit it can reduce the quality (the tree can make only so much sugar, and it has to be distributed between all of the fruit). Citrus trees also tend to drop some fruit in late summer as they make the final adjustment to the crop.
Stress can also cause a tree to drop fruit. High populations of insects like whiteflies or some diseases can stress the tree and cause additional fruit drop. Weather conditions like drought or excessive rainfall can also stress trees and add to the normal fruit drop.
Sounds like your trees are in good shape and are likely holding on to all the fruit they can. All the rain is not helping things.
Consumer Horticulture Specialist
The LSU AgCenter and the LSU College of Agriculture