Many things can cause banding. Banding can be caused by the program that it is exported from, such as Indesign or Corel. Also, too many gradient steps, for example going from a very light color to a dark color, in a small area will cause banding.
CLOSE UP OF BANDING
To prevent this, check your digital files before sending. If you use a gradient, make sure it has enough room for a smooth transition.