My outside water faucet runs normally when I turn it on, but when I connect a garden hose to it, the water starts to back up in the pipe and leak into the basement. Why?
It sounds like your hose bib froze during the winter and you have a crack in the pipe from the faucet. You probably have a frost-proof sill cock, which actually has a shut off valve inside the house, through a long stem from the handle. The crack is probably tight enough to prevent water from leaking when there is no back-pressure, but the moment you put a hose on and increase the back-pressure it leaks into the house. You will probably need to install a new sill cock.