It may be that the previous owners did not have the tank pumped often enough. This can cause some solids to be flushed into the leech field. This is not good, since the field is designed only for liquids. Get too much down there and you can quickly clog it and require a replacement. Sound like there was a clog there that the knocking dislodged, pushing it further down the slope. Depending on how far, from that point down, it is probably still blocked. The best practice is to get the tank pumped regularly. This somewhat depends on the number of people, the size of the tank, and how consciences you are about what you put down the drain. Somebody that works with these things regularly can better help with details.