Hair distribution is something that is different for each person and reflects a complex balance between genetics and hormonal influences. While general patterns of hair growth are established after puberty, new and significant hair growth as an adult can reflect an underlying process affecting overall hormonal balance and/or metabolism. For this reason, it would be a good idea to make an appointment with your primary care provider so that he or she can examine you and also take a thorough history of any other symptoms you may have developed as well as any new medications or other changes in your life.
You didn't mention whether you are a man or woman, but this can be an important starting point for evaluating new hair growth. Many men do have hair on their back, and this would be a less concerning symptom in a male patient than it would be in a female patient. However, whether male or female, any changes in the production and regulation of sex hormones (involving the hypothalamus and pituitary in the brain; the ovaries or testes; and the adrenal glands) can cause subtle changes enough to alter hair distribution. Problems with the thyroid gland can also alter hair patterns, as can several medications. Again, please discuss this with your primary care doctor