Gardeners in the Midwest expect crapemyrtles to die back to the ground each winter. In the northern part of their range, they are root hardy. I’m not certain why yours in Alabama have so much die back. Are you having late freezes that are knocking them back? You might check in with your local garden center or extension agent. These local experts may be able to answer your question better than I can since they're more familiar with your weather and conditions.
Thanks for writing!