Other Crape myrtles in my garden have exfoliating bark but the underneath is light colored.
Various theories expressed as to the reasons besides the bark getting too small for the tree include increased transpiration of water in a wet summer like we just had and a method of getting rid of parasites, fungi, and mosses and lichens. This last theory is illustrated in the above photo. Young Crapes sometimes have heavy growths of lichen, shed with the exfoliating bark.
These last two below are exfoliating bark of Loropetalums. They look as though they just outgrew their bark.
Looking at these pictures remind me of the Victorians describing arms and legs as 'limbs.' Have you noticed exfoliating bark in your garden?