This happens due to the safety valve which acts as a safety feature. When pressure and temperature inside the tank increase above the safety measures, the appliance leaks pressure and water through the valves. This prevents the system from malfunctioning. However, if your geyser is overflowing from some other place, instead of the safety valve, then there is a problem with your appliance. For example, the inner tank can have a leakage. In this case, you should call for professional help to get it fixed.