The bathtub may be one of the lower drains in your house. If you do not have a floor drain in your house, the bathtub is likely the next lowest drain. When you run water in the sink or flush the toilet, water running into the tub is a sign of a serious blockage in your drains or sewer. You should call a sewer & drain professional to come evaluate the situation and make corrections.
Radlerooter Sewer & Drain Cleaning