Probably water deficit and extreme drought stress. The next stage of decline may be defoliation, branch die-back and mortality.
Soil and water management are vital. Drip systems are often not enough, especially if not maintained and adjusted to the extremely hot dry season there. Deep watering once a week, mulch and fertilization will keep your crape myrtle healthy, if it's not too late.
If you think this educated guess might be an accurate assumption, go out and water it today. Not just at the base of the tree trunk, but in wide area of the soil surface at least as far as the spread of the foliage, and more.