I crawled down and gave the valves a good examination, and it looked as though some of the bleeder screws on top of the valves were loose. I also found that the one controlling the line to the rootstock table had broken off completely. I tightened everything down, and put one of the drip emitters in an empty bucket to detect any more unauthorized irrigation. Hopefully the bucket will still be empty when I check it in the morning, and my side yard will start to dry out. I had to take some drastic measures to control all that extra water, but that's a story for another blog post.