SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Simply sounds like the backflow device was doing what it's supposed to do.

My guess is that while the water company was "playing" with the water supply, the water pressure a times dropped. When that occured, the water in your irrigation pipes between the backflow device and the valves were at a higher pressure than the water supply, so the water was trying to flow backwards into the water supply. But the backflow prevented this from happening.

The only thing to possibly concern yourself with is that the spitting of water out of the backflow device is supposed to be a fail-safe. Basically, even if the check valves in the device become blocked by debrie, the device should still prevent the back flow of water by allowing water to exit from the relieve valve. Now I could see how rapid fluctuations in source pressure might cause this fail-safe to kick in, but it might have also been caused by trash that has been lodged in the device (from what I've seen, strainers are supposed to be installed up-stream of these devices to prevent such stuff).

If you have a high supply pressure, some RPZs (post a photo of your backflow preventer) can dump water after a water-hammer occurence sets up a condition of oscillation.