Even if a pressure reducer failed, it would be unlikely to result in pressure high enough to trigger a T&P. Usually a drip means that either the valve is bad, or you have thermal expansion problems. You can test for the latter by getting a water pressure gauge and putting it on the water heater drain valve or any cold-water hose bibb, making sure all hot faucets are off, and then making the tank fire by turning up the thermostat. If pressure goes anything above 80 psi, it’s thermal expansion. The fix for that is an expansion tank.