I wonder how often you are watering these plants. If they are interplanted with azaleas, you may be giving them more water than they need, and possibly having a problem with powdery mildew. If you have a white powdery growth on the leaves, this is powdery mildew. Heavy infestations can cause premature leaf drop. Aphids are also common on crape myrtles and can cause a build-up of black sooty mold, but is more unsightly that anything.
Crape myrtles thrive in full sun with dry conditions.
If yours are still coming back strong and blooming every year, it may not be a problem. Check to see if anything has been sprayed nearby which could be causing the early leaf shed. Back to top I have two large crape myrtles and several dwarf. They are watermelon color and normally bloom from July until fall. This year, they bloomed in July and ceased.