I have a drip system running to my vegetable garden that's connected to an automatic sprinkler system. This was originally a sprinkler system for lawn but I swapped out the sprinkler heads with drip heads when we converted the lawn to raised garden beds.

Before winter and after I've pulled out the last of my vegetables, I unscrew the drip heads and cap off the risers. I've had this system in place for 5 years and have never had a problem until this year. I noticed after we've had some freezing night temps (into the high 20's) one of my empty garden beds looked like it had been soaked overnight. I dug out the sprinkler line and found it had come disconnected at the elbow joint where my 3/4" PVC turns to come up out of the ground. I left it unconnected and the hole open so I could monitor it for a while. The hole dried up and stayed that way for two weeks until last night when our warmer nights cooled down again and we had more freezing temps. I can't catch it in the act, but at some point during the night or early morning, water had gushed from the line, enough to fill up the hole (it's about 2' deep) and spill a river down my garden bed.

What I don't understand is that my system is turned off, so why would I have an intermittent gush of water come out? I've done a little reading about winterizing sprinkler systems, something I've never had to do here in California. Some info suggests I might have a leaky valve, but if that were the case, I would have expected a constant flow. This seems to go from nothing to a geyser during a cold night then nothing again.

Any ideas on what might be going on?

Thanks in advance for any help you can provide!