Maybe, maybe not. If you have an expansion tank, it may be shot. If you don't have one, maybe you need one. Sometimes when the valves get old, they start to leak, but it is designed to release water if the pressure gets too high, or the temperature gets too high.
Now, you ask why the pressure would be high...heating water expands it. Since the pipes are basically pretty strong, there's no place for it to go except push back out to the street, or if it can't do that, out the weakest point which is usually the relief valve.
To provide a place for the water to expand into, you use (ta-da) an expansion tank. This is basically a tank with a flexible membrane where one side is filled up with pressurized air, and the other is connected to the cold water supply. When the WH runs and the valves are off, it expands into the expansion tank, keeping the pressure fairly constant. When you open a valve, the bladder pushes that water out, and the water flows normally.
A pressure reduction valve will block off the water from going back out to the street as will a check valve. Some water meters have one in them as well. So, if one of these exists in your system, when the water expands, there's no place for it to go, and the valve leaks until the pressure is reduced.