Question: Why is sewage backing up into my lower level toilet?
Answer: If sewage backs up onto your lower level toilet or a lower level drain, you probably have a sewer line blockage. In most cases that means you had a crack in your sewer line that has allowed roots to intrude. Tree roots are always seeking water and nourishment, and any crack in a sewer line will attract them. Tree roots are incredibly strong; they can easily pry open a cracked sewer line. Once tree roots gain entrance, they grow into the sewer line, blocking it completely.
If this is happening, you need a plumber to use a sewer line camera to determine where the blockage is. That part of the sewer line will most likely need to be replaced.