Your electric water heater uses a TPR (Temperature Pressure Relief) valve. This valve is basically a safety valve used to release the pressure that forms in the water heater. When the pressure rises, the relief valve will release some water and steam into the air. If the relief valve stops working, you can run the risk of over pressurizing your water heater and potentially causing a blow up. Below are a few problems that may cause a TPR valve to leak.
1 - Defective Valve
If your relief valve has worked perfectly for a very long time and only suddenly has started to leak, then your valve may be defective due to age or general wear and tear. The seals just may no longer be working properly.
2 - Stuck in Open Spot
Again, due to aging, the relief valve may just have become stuck. If this is the case, simply shut off the water and replace the valve.
3 - High Tank Pressure
One other related problem to a leaky TPR valve is high tank pressure. You may have a problem with the check valve or the pressure on your main may just be too high. Check both elements.