Hair growth in men and women is governed by hormones called androgens. Specifically, androgens are the hormones that cause the development of "masculine" features, both in men and women. One of the characteristics that androgens are able to affect are hair follicles. Hair follicles have small proteins (called receptors) that are able to recognize and bind to androgens. This causes the hair follicle to be stimulated to grow hair. When androgens begin to circulate (often in men during puberty), body hair will begin to grow. It will first grow in the areas that are most sensitive to androgen, such as the pubic region, but over time hair can develop in other areas, such as the back.
One cause of back hair growth in men is a condition known as hypertrichosis. According to the website Hypertrichosis.com, hypertrichosis is a condition in which there is abnormal hair density and length. It primarily affects men, and this abnormal hair growth can occur anywhere in the body, including the back. Hypertrichosis is due to genetic abnormalities that can be inherited from the parents. It causes hair follicles to be abnormally distributed or to respond in an unusual way to normal androgen levels. Hypertrichosis is generally not an actual medical problem and is instead a cosmetic one. Hypertrichosis rarely causes any significant medical problems or complications.
A potentially more serious cause of back hair growth is hirsutism. Hirsutism is a condition in which the levels of androgens become elevated. Because the hair follicles respond to androgens, this can cause unusual hair growth, including on the back. Hirsutism, as opposed to hypertrichosis, is potentially a very dangerous condition because androgens can have many effects on the body, such as weight gain and irregular menstrual cycles. Hirsutism is usually a problem with the production of androgen, which can often signal a more serious underlying condition, including cancer.