I have a gas water heater (50 gallon) in the garage. There are three openings on top of the tank (roof or the top surface of the tank). One is for cold water pipe, one for hot water pipe and the third one is a valve with flip style. Today I noticed water on the floor of the garage. Hot water was coming out of the valve. When I lift the valve (it is a flip style valve) more water comes out of the valve. Any idea what is causing the problem? Thanks

They go bad, BUT

If you have a pressure reducing valve or backflow preventer or high water pressure (>55 PSI), then these issues need to be addressed.

Expansion tank or regulator may be necessary.

You can purchase an inexpensive hose bib pressure gauge.