Have you recently installed a hose on a 2- handled/valved faucet and left the valves on the faucet open/on and only using the valve on your hose for control? That is the number one condition we use to see when I was involved with commercial plumbing company. All faucets on modern fixtures have check valves internal to the valve to prevent this but some of the older faucets you may find on a service sink that may be in a garage or basement may have an older faucet that does not have this protection. When you put a hose on it and leave the valves open, only using the spray valve for control you have basically connected you hot and cold water lines together.
We have this all the time at work. Both valves on a service sink are left open to a sprayer. Sometimes the kitchen sink. Do you have a sprayer on your kitchen sink. How long has it been doing this?
