When it rains heavily, septic systems act in the reverse and water runs into the house. If your lower level doesn't have a backwater valve attached to it, this can be a problem. Imagine you are going to install the distribution lines (leech field) on a septic tank. You dig a trench about 18" wide and 24 inches deep. You install a pipe that has holes in it and fill the area with gravel. All the ground around this new trench is undisturbed compacted soil. As soon as it rains, the trench accumulates the water and it backflushes into the septic tank. If the pressure becomes too much, the water runs up out of the toilet and floor drains. If your ground is just too moist with a septic tank, you will get very little drainage. Then the water that you flush from your up stairs toilets will come out in your lower level, since the ground can no longer absorb it. Its like when you get a sponge too wet, it will no no longer pick up spills til you ring it out. The ground has to be dry for a septic tank to work. After a few days the ground is drying out.
The reason the other fixtures work, is because they drain much more slowly than a toilet. A toilet is an instant flush of a large volume of water. A couple of problems happen with a septic, that is full because of groundwater. Toilets don't want to flush. When you try to push a large volume of water into a container already full, there is no place for the water to go, because there is air already in the line. Suppose you put a straw in a glass of water with your finger on the end, the straw will remain empty, because the air has volume. If you remove your finger, the water will enter. The water in the drainfield is like a finger on the end of the straw, it forms a plug. Therefore, the air inside the pipes resists the water trying to enter in a large volume. Whereas the water entering from a small pipe, enters along the side of the pipe and exchanges places with the air, that heads up to the roof.
Raymond VinZant Plumbing Prof.