The common cause of lines appearing down the page in laser printers is typically related to a defect in the imaging drum. Over time, with constant use, the photosensitive drum may be scratched or damaged, which results in lines on the printed output. Replacing the imaging drum is the solution to this issue. A dirty corona wire, while it can cause print quality issues, typically causes a faint print, rather than distinct lines. Printer calibration is not related to causing lines but is more about color and alignment issues. An incorrect paper type could cause jams or poor-quality prints but would not specifically cause lines down the pages.