Moire on print

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