A lot of people don’t realize that once a pipe has been inside the ground for so many years it is dried out of its moisture and things begin to go wrong, hence we have root intrusion. To educate you, roots only want water and nutrients, if they could find it they will follow any type of source or opening that is available. And when you have dried out pipes brittle and worn down they will leak the sewage water into the soil and the trees can smell that water and waste. Watch the trees smell that water touching the dirt Underground they send the roots to the broken source to gather as much water as possible. The Roots find the break in the pipe, it enters the pipe by growing stage-by-stage, and once it is inside the pipe it gets stronger and stronger and stronger. And the stronger it gets the bigger it gets the more obstruction that would be in the pipe, this would cause a blockage for sure once it reaches a certain Mass. The photo that you see attached to this post is a commercial grade sewer line and this is one type of buildup, as you can see there has to be a combination of grease, dirt ,film and Etc… Preventive Maintenance is the way to go.