Genetics is the most common cause of a hairy back in men. Certain genes in men make them more likely to be susceptible to the effects of testosterone, which encourages the growth of body hair. This is also why most men grow facial hair more easily than most women.
There’s another reason as to why you might have a hairy back. Hypertrichosis is a disorder which causes extreme hair growth all over the body - but try not to worry about this one too much, as it is very rare. If you think that you might have hypertrichosis, however, it's best to consult your doctor for further advice.
Check out our section which is focused on answering all of your body hair questions here: The Truth About Male Body Hair.