I think it's a combo of genetics and about any type of psychological component.
My AS was never raised around chronic alcohol use so he's got little or no background there. His biological father (whom he only saw on weekends for maybe 2 years out of his life) and paternal grandfather were both drunks, in fact the grandfather died from his disease before my son was ever born. On my side of the family, my mom is a functional alcoholic, but likewise, he wasn't raised around her and she's never been a sloppy or uncontrolled drunk. No nurture vs. nature argument here.
I sincerely believe that my son became addicted to alcohol while he was trying to self-medicate for depression and his genetics took control. He had a lot of money issues and relationship issues about 3 years back. In three years he's become a card carrying drunk.