Water that's coming out of your tap is generally aiming for neutral (~7 ph). The alkaline waters in the store are using minerals to elevate it into probably the 9s.
Like many hippie health trends that then go mainstream, a lot of it seems to filter in from Rastafarian traditions, where in the Ital diet they believe that alkaline water coming from the Blue Mountains in Jamaica (and other natural springs there which are naturally alkaline) help detoxify the acids in the body.