It is difficult to say exactly what is happening to that crape myrtle. It could be excessive soil moisture causing the problem because those plants don't like to have their roots sitting in soggy wet conditions for extended periods of time. It may also be that it was planted to deep but you should have seen problems than just last year if this was the cause. It also may be viewed to a root rot diseases such as cotton rot. There is no cure for this so you'll just have to wait and see and hope for the best regarding cotton root rot possibilities. In the meantime keep the plant adequately watered but not soggy wet. Your current watery schedule sounds about right but just make sure that it is providing a good soaking and then allowing the soils to dry out moderately watery again.
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