Moire on print

[generalbatzorig]

What causes moire on printed images?
The effect is not in the digital images, so they are definitely the result of the press. They are on some (not all) pages with grayish neutral backgrounds. I am suspecting the lines are caused by the screen angles. Although they are very fine lines at around 100 degree incline, I can still see them with my naked eyes and the print looks bad. Here is a link to an example scanned image from my book.

https://flic.kr/p/2qVoVj5
https://flic.kr/p/2qVoCSZ

 

[SteveSuffRIT]

You would not "see" a moire pattern on a computer screen because the image is created with pixels.
Also, you would not see a moire in a hardcopy inkjet proof because that is nearly continuous tone.
Printing with halftone dots creates a "rosette" pattern of the angles are correct and in register.
If incorrect angles (CMK not separated by 30') or out of register, you can get a moire pattern.
If your gray background is made with CMYK, I would replace with maximum Gray Component Replacement (GCR) so there is mostly/only K present. Fewer colors is less opportunity for moire.
Best solution is to print with FM spots (random) instead of AM dots (pattern).
Pattern + Pattern = Pattern, moire bad, rosette acceptable
Random + Random = Random.