That is a common stress related symptom on crape myrtle. It may be due to dry conditions or even soggy conditions or fluctations between the two. Heat is also a factor when combined with the long summer season. In other words, at the end of summer as we approach fall some plants when still stressed will begin their fall leaf drop. A cool break with some rain will usually stop this at least until the real fall arrives! I would not worry about it much except to note that in future summers some good waterings of a large area around each plant on an infrequent basis can help.
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