There aren't rollers on dtg like laser printers, etc. Some of the 48xx based systems still have rollers, but they are behind the printhead, so if you do more than one pass, this could happen. What you are seeing on this image is not that problem. One reason is, it's on the outside area of the graphic, so no roller would be touching to cause it and two, the lines are not straight and more of a loose pattern. To me, this would indicate the artwork.
Look at the artwork close to see if you have stray pixels in those areas. You can take it in Photoshop or a similar program and place a black background on the image, it would show you any pixels in unwanted areas.
If you can't see anything there, try a different image to see if you are still having the same problem. If so, you could check your encoder strip to see if it needs cleaned. If it was an encoder strip issue, you would typically have a more defined pattern than what you are seeing here.