Depending on how your rig is plumbed up, what others have shared regarding a leaky valve or check valve, that would be the cause.
A leaking check valve can be the result of a dirty valve ball and/or ball seat. You could try bleaching & flushing your system to clean it.
On rare occasion something physical may have gotten lodged in the check valve. People have had trouble with tiny plastic debris left from when holes were drilled in the fresh water tank during the construction of the rig. It can take many years to get into bad places.
Sometimes that debris can be sent on it's way by running the on-board pump. Just be sure to take off the running faucet's aerator to let the debris out.
