You get a bit more speed out of the C64 if you disable parts of the video chip. It also avoids cycle inconsistencies every 8 scanlines (where normally, the chip would block the bus for an additional cycle).

To avoid making it a dull monochrome screen, devs tend to push the data that they process into the border color register (acting as background color in this mode). You also see this with decompressors at the beginning of (cracked) games. So the background color gets updated every few multiple of 8 pixels (1 character), and there's where the patterns come from. The slow shift of the pattern to the left indicates that the code isn't entirely balanced, but the article states as much (is it moving every 1.28 secs? I can't tell).

They're deliberate, done by incrementing border color register. One of the only things possible with the available CPU time.

They're raster lines. At some point in the code, or multiple points, the program increments the register holding the border color so the video chip starts drawing a different color. This can happen in the middle of a line, which is why you get multiple "columns".

Edit: vardump is right, the screen would be "turned off" in this case so just the border color is used. This gives a slight performance boost.