What causes a backup in sewer line?

Any thing from hair to toys to roots or anything you jam down your toilet or drain pipe that wont decompose. I have snaked out a lot of sewer lines and I have to say that the number one reason for a clogged drain line is the pipe is not graded correctly or is back graded from settling. My 2nd most reason is tree roots or damaged pipe. If your home has plastic pipe like DWV ABS or PVC and it all grades perfectly down hill without damaged pipes the chances of having a clog are very very slim. Kitchen drain with disposals are the only drain lines I personally seen that have clogged with perfect grade with plastic drain pipes. Shower and tub drains mostly clog at the shower/tub strainer or trap and very rare clog after that if its plastic pipe and has slope.