The tree is attempting to replace all the tissue that was pruned away. Instead of the usual spacing of shoots that would normally occur, there is a proliferation of shoot development from the awakened dormant buds. You may ask why this is important. Normal annual growth is regulated by the plant and is a response to the growing environment. This includes the amount of water, nutrients and sunlight.
Plant growth is a never-ending attempt by the plant to balance shoot growth with root growth.
Severe pruning throws that delicate balance off. The plant has to rely on stored energy reserves to reproduce all the wood and leaves it lost. The plant becomes more stressed and susceptible to disease. There is a net loss of energy. If crape myrtles are severely pruned this way year after year, the shoots become smaller and smaller each year as the energy reserves become less and less.