Q.Why do my crepe myrtles die back every year?
I have a row of crepe myrtles spaced several feet apart and they die to the ground every year and send up new growth from the ground each spring to a height of three feet. Not knowing there were different sizes to myrtles, I assumed these were shrub size. Last year I grew Muskogee 2ft. from cuttings and planted them. Same thing, died to the ground. I want trees…what am I doing wrong? Zone 7 with amended clay soil in full sun.

The pattern you are describing is common if the Crepe Myrtles have been over pruned or improperly pruned.
You can try this to help them develop.
Pick 3 to 5 of the strongest limbs and let them grow.
Cut all the rest down and keep the suckers trimmed back.
After they are a good height, limb them up to to a good height and so you will have a smooth trunk.
Don't limb them up to soon though, as they need the leaves to make food and grow well.
If they are dying back to the ground it may have been to cold for them.
Here are some links with more information.
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/pruning-crepe-myrtle-trees.htm
http://hort.ifas.ufl.edu/database/documents/pdf/tree_fact_sheets/lagxd.pdf