This was done by scanning a completely blank page first, then printing the result, scanning it again, et cetera. The result you see was achieved after about five repetitions of the cycle.
What, right after the second round of the process, arises, are Moiré patterns. I think this is because the scanning process produces errors in a linear fashion, row after row, and this results in interference when the errors add up.
By the way I think scanners are wonderful stuff and you should go play with them whenever you can. A really easy but kinda fun project is to get people’s fingerprints and enlarge them, this can be done with grafite from a pencil, scotch tape and, of course, a scanner.
See my previous project where I scanned my laptop’s screen while playing a video.