The short answer is 'your brain causes it by assuming patterns to exist which sometimes do not, and then it makes you actually see them.'

The dullish green/brown lines are close enough in color to the white and black squares of noise, such that it is sort of like what you'd see in a low light environment.

Then, the noise squares themselves do have faint traces of arcs in them, and your brain is quite good at extrapolating / inventing details that are not directly in the center of your vision.

To me this looks like an overdeveloped pattern resolution process aimed at making better sense of a low light, night time situation.

Better to get spooked at nothing, occasionally than to not at all see some kind of night time predator.

There are even some night time camouflage military patterns that make use of this 'misinterpreted grid' effect, here is an example:

https://en.m.wikipedia.org/wiki/Desert_Night_Camouflage

As the wiki notes, it was designed to be used against early night vision devices, but as NVGs have improved greatly, it is no longer used.

There seem to be curved, gray "lines" throughout the picture, like this one in the bottom center. Perhaps this confuses your eye, since the resolution of anything outside the center of your vision is terrible and mostly your brain's best guess? Also color vision gets progressively worse due to fewer and fewer cones/rods (I don't know which one was which) outside the center of your vision, so maybe your brain literally cannot properly distinguish the green lines from the gray?

Assuming you're not viewing Lemmy on a CRT, the lines are indeed reasonably straight lol