It's pretty common to find a water heater installed by non professionals. It's also very common to find incorrect installation of the water heater TPR (Temperature Pressure Relief) In this case the tube for the valve runs uphill when leaving the valve. If the valve starts going bad or gets some dirt in it, and starts to drip a little water, the water can't go anywhere. It will sit in the line until enough pressure builds up to blow it out. This can create an unsafe condition and I have read stories about valve problems and the water heater can literally blow up and out!
When this valve starts to go bad you may see water dripping from the end of the tube. If it expels water it can be very hot & dangerous causing severe burns. In extreme cases it's can blow out a ceiling! Check out this episode of Myth Busters and experimenting with a TPR valve & water heater