Someone asked this question a couple of weeks ago. Search the forums for
"Watts Series 800M4QT Anti-Siphon Pressure Vacuum Breaker Cap Leaking"
That 'cap' prevents water from flowing backward into the water supply from the hose. The cap should unscrew and you might be able to detect a crack or bad gasket that is preventing the valve from closing properly.
I have a cheap, plastic bib that cracked in the winter and I have the same pblm as you.