Water Release Valve that doesn't look like a Pressure Release Valve
That looks EXACTLY like a pressure (only) release valve (as opposed to a pressure/temperature release valve.) I have one that looks like it on the inlet of my water system, though I do have an extender pipe to take the water nearer the floor attached to it.

As far as I recall (I tested when I installed it, but that's a few years ago now), mine resets on its own when the pressure is reduced below its threshold (it's just a spring seal) so I would guess that whatever  caused the part of the system that is being protected by this valve to be over-pressure was stuck there until you turned off the main valve. Identifying what that was and fixing it so it does not stick open would be my best guess on not having this happen again, but it might not be easy unless you have a pretty good idea of what it was (you suspect the irrigation system, presumably for some reason.)