Sounds like Henry is right on with the PRV(pressure reducing valve). When those are installed because of high city pressure, you also need to install an expansion tank on the inlet side of the water heater.
It also sounds like your landlord did the work instead of a licensed plumber since it is a sears wh. Luckily sears also sells the expansion tanks. If you have a prv that was opened up, the increase in pressure plus the pressure from the water in the tank heating up causes the increase in pressure to exceed you t&p popoff valve on the wh.
You may have gotten by without one before, but with the new wh, I wouldn't press your luck. Get an expansion tank. Good luck.