Of course a broken pipe is possible, but I suspect you have an Apollo shut off valve like mine. If so, it takes 1/4 turn to open/close the valve. That's simple enough, but there is no stop on the valve so you can pass the stop point. What will happen then is the drain feature called, "Stop and Waste" will begin to let water out. If you turn the valve clear off, the pipe above the valve will just drain and that's the end of it. But if you get in between off and on, the water will still enter the system and the stop and waste will try to drain. I hate this damn valve, but it's 5' underground and it darn near kill me digging that pit when I repaired a broken pipe and replaced that valve about 5 years ago and I'm just living with it. Next time, if there is one, I'll spend the extra buck and get a Muller valve that has stops.