I had this same problem, but it was gray water. Looking under the trailer, I noted the piping running from the gray water tank to the discharge valve, about 6' of pipe, was higher in the center than either end. This caused to tank to not drain completely, then when the trair was moved, the water would drain from the tank and become trapped at the discharge valve. While the trailer was at the dealer for other repairs, I brought to their attention. They repositioned the piping and no more problems.