What is happening is there is a break in the sewer main and dirt is getting into the pipe. The dirt is either causing a blockage (or something else is) and it's causing your sewer main to back up. The backing up water carries the dirt along with it.
I've seen this a few times out in the field.
You need to get someone out there with an inspection camera and locate the break. It'll have to be dug up and fixed! Just pray it's in a good spot!!!