Shift Marks are created in with width of stepover stitch loop-pile carpet. It occurs from the resident vibrations in the tufting process. If the rate of production is increased, then the variations in the lines may change.
They are are not always a manufacturing defect. They are most noticeable where the lines do not line up at seams.
Below is a picture of a loop pile carpet with shift mark. This shot is looking at the carpet in its width, thus you can not see the lines.
This shot is of the same carpet, but looking at its width. Now you can see the lines.
Here is a link to an old report for a carpet mill on this topic.