Shrubs and trees, seeking moisture, will make their way into sewer line cracks. These roots can do a lot of damage. They may start out small, getting into a small crack in the pipe; but as the tree or shrub continues to grow, so does the root. After time goes on, this obviously causes your sewer line to break, which in turn allows debris to hang up in the sewer line, thus causing a backup.
One way to prevent roots from entering your line is to replace your line and tap with new plastic pipe. The other alternative is to be careful about planting greenery around you sewer line.