I would venture a guess that either: the pump is not working and sewage in not being pumped out. Pumps usually do not work intermittantly, but floats can get caught and cause the pump to run continually, but sewage is pumped when this happens so this is an unlikely cause. Or the volume of sewage that is being put into the sump is too much for the pump to handle--it is running continuously, but sewage is not being ejected as fast as it is coming in. Or maybe a partial clogged drainage line clog could cause this too--not pumping out as fast as it is inflowing. If you are getting sewage into your unit when you are not using fixtures, this is most likely in a main line somewhere and may actually not be your responsibility alone, you could just be the one feeling the (nasty) effects. There should be a check valve to keep other sewage out, but these can fail or sometimes they are not there.