This is moiré. It occurs because a screen is actually a grid of squares that are being used to make the image. When it ends up trying to be mapped to another grid of pixels (either by being captured by a sensor or by scaling) points of light or pixel data don't line up exactly. Some pixels get 2 pixels of information, some get the border between pixels. The Wikipedia article I linked to has far more detail available on it. It can happen any time that two grids interact (such as taking a picture of a skyscraper from a distance.

In the case of scaling particularly, if you have multiple pixels per screen display pixel, the actual effective resolution of your image is limited to whatever you were taking a photo of (there isn't more than about 2MP of information on most computer displays). What happens with the extra pixels on your camera is that they actually take an image of what the screen looks like, so you get multiple pixels for each screen pixel and some that overlap borders.

When you scale that down, particularly depending on the algorithm used, you can end up with moiré emerging because pixels that were on the border between pixels get too much weight, resulting in darker areas. This actually happens a little bit with any kind of remap, but if it isn't two grids of evenly spaced points, you don't get enough of a pattern for it to be super noticeable like it is in your sample image.