If the water is warm, Terry's suggestion is probably right. If not, it will take more investigation.
Some places put in a pressure relief valve to bleed off excess pressure if it gets too high. I'm not a fan unless there's something critical you need to protect, but if it was from that, it would likely be cold. In either case, it might be a good idea to check to see what your water pressure actually is, and that's easy. Pick up a water pressure gauge with a hose connection on it, and screw it onto say that hose bib nearby. If you get one with a second, tattle-tale hand, leave it there for 24-hours, to capture the peak water pressure, which may be in the early morning hours when nobody in the system is using much water, and they're refilling water towers.