You might check your drain vent, if you have one. This is usually a tube visible on the roof, and a few helpful youtube videos are out there describing how to fix the issue. Basically, the drain vent provides a passage for air, to help leverage the movement of water through your pipes. If that air doesn't come, then water through pipes creates pressure and (surprise) water can flow backward, coming out of drains elsewhere in the home.
You might check your drain vent, if you have one. This is usually a tube visible on the roof, and a few helpful youtube videos are out there describing how to fix the issue. Basically, the drain vent provides a passage for air, to help leverage the movement of water through your pipes. If that air doesn't come, then water through pipes creates pressure and (surprise) water can flow backward, coming out of drains elsewhere in the home.