Sewer back-ups are caused by several factors including sags in the line, line displacements, tree roots, debris in the line, cracks in the line, line collapse, and deformed pipe. It is impossible to know for certain what might be the cause of your line backing up. The best approach is to send a special camera down the line to see what is happening. I have a camera for this purpose which I run from the cleanout in the home to the city street. The camera is crystal clear so long as the line is not full of water. If you have a back-up it is best to auger the line first then call for the camera. My equipment has a built in DVD burner and I also have a line locator that allows me to find the exact spot where the problem is located. Check out the video on my web site for more information.