Pattern in scaled down image of very close lines
The moiré fringes are caused by undersampling, it is the difference between properly sampling a small field of view and undersampling a larger field, it causes spatial aliasing.
This seems to be an effect of aliasing. Basically, the diagonal lines are drawn with discrete pixels, and then the width of the line varies as it is rounded back and forth between successive integer number of pixels.
This rounding occurs differently depending on the width of the line at that point and the angle of the line, but since it occurs systematically, you can sometimes see patterns appear when you look at the picture as a whole.