Hi Greg, I've checked several reference sites and all point to the need for an expansion tank on the cold-water inlet. This is not uncommon and may be the result of increased city pressure, a water main regulator adjustment, or a lower rated or more sensitive TRR valve than on your previous heater. Many older TPR valves get stuck from mineral deposits and don't work properly. Here is an explanation I gleaned from another site. I hope it helps and Good Luck. (From the website of Chili Pepper Appliance Company, www.chilipepperapp.com) TP relief valve leaking water.
The relief valve will operate if either the water temperature is to high or if the pressure gets too high. If the inlet to the water heater is fed directly from the water main with no pressure relief valves or check valves between them then when the water heater heats the water and it expands, the extra volume of water simply flows back toward the water main. If however there is a blockage such as a check valve or pressure reducing valve with a defective bypass then the increase in water volume has no where to go and the pressure with increase dramatically. Placing an expansion tank in the line at the inlet will give the increased volume of water someplace to go and prevent operation of the relief valve.
Another possibility is that the City water pressure has increased above the setting of the relief valve. Again the installation of an expansion tank will solve the problem.