The tree seems to be suffering some type of root stress. A lack of water is the most likely, although too much water is also stressful. If they are in a clay type soil the planting hole can become an underground bathtub and the roots start to drown for lack of oxygen when the plant is overwatered. Fertilizers are salt based in many cases and can burn roots when overapplied causing leaves to brown.
I suggest you evaluate the soil moisture a few inches down below the surface and water accordingly. Next season try to keep the soil evenly moist and fertilize lightly using a lawn fertilizer product at a rate of about a half cup spread evenly in a 5 foot area around the crape myrtle.
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