That valve is designed to open if the pressure exceeds 150psi, OR the temperature exceeds (I think) 212-degrees. If it was dumping cold water, it certainly isn't from excessive temperature.
If it opened periodically for some reason, it may have some mineral deposits on it now that have prevented it from closing, but something opened it up in the first place.
You can buy a water pressure gauge at a big box store or plumbing supply. I'd pick one up for about $10 and actually check the pressure. Get one with a second, tattle-tale hand to show peak pressure and leave it on for at least a 24-hour period.
You indicated that the local pressure might get to 90psi, and codes generally require a PRV and an expansion tank to limit it to 80psi. Also, keep in mind that if your static pressure is 90psi, in the early morning, late night situation when few people are using water, it could peak a lot higher while they refill water towers, and because the demand is low.
If the pressure and temp are below the safety limits, the valve may be defective and should be replaced. This will take some plumbing skills, as the thing needs to be unscrewed from the tank and the drain line from it will need to be removed and a new one soldered into it because that would normally prevent you from unscrewing it with that long pipe on the end.
If the thing wasn't installed properly and the discharge wetted the gas control valve, then the whole WH should be replaced...a wetted gas valve may never work right again and is a safety hazard.