Abnormal spinal curves develop for reasons that are largely unknown. Although there are several theories, about 80% of scoliosis is considered idiopathic (of unknown cause).
Statistically, about 4 adolescent girls in 1000 may develop it, and about 1 in 2500 boys. Roughly 2% of the adult population has some degree of it.
In my practice, I have found, in working with several adolescent girls, that a combination of stabilizing the pelvis & focused exercises can reverse or halt the progress of scoliotic curvatures.