A plumbing leak is a common problem when turning on the water heater and excess pressure builds up if there is nothing to absorb the expansion of the water from heating.
The plumbing system is designed to have an air "bubble" in the top of the water heater to take up this thermal expansion. This air bubble can be lost from air forced or let out of the temperature/pressure relief valve on the water heater, or simply absorbed by the water over time.
You need to restore the air in the top of the water heater if it has been lost. Try this first, it's often enough to cure various plumbing leaks caused by excess pressure. The article is from Airstream and was published by WBCCI in their magazine. Very simple procedure.