The answer to 'why is my water heater overflowing' is that it is likely due to a problem with the pressure relief valve or the tank itself. Overflowing can occur when the water pressure or temperature inside the tank gets too high, causing the water to back up and spill out.
Some common causes include a faulty or blocked pressure relief valve, a problem with the thermostat or heating element, or a buildup of sediment in the tank. It's important to address the issue quickly, as an overflowing water heater can lead to water damage and other problems.