What are these rectangular patterns in the frame?
Those are compression artifacts, where the original video has been compressed and has lost data. Block-based coding for quantisation leads to artifacts like this, as the information depth in the image is too high to be successfully compressed at the level required by the lossy algorithm used. From that Wikipedia page:


  Lossy data compression involves discarding some of the media's data so that it becomes simplified enough to be stored within the desired disk space or be transmitted (or streamed) within the bandwidth limitations (known as a data rate or bit rate for media that is streamed). If the compressor could not reproduce enough data in the compressed version to reproduce the original, the result is a diminishing of quality, or introduction of artifacts. Alternatively, the compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the viewer.


@DarkDust worded this well:


  The part that actually leads to these visible blocks is the fact that for some compression algorithms, each frame is divided into smaller parts, so-called macroblocks, which are further divided into transform blocks. When the compression in two adjacent macro blocks is too far away from the original in different ways, they don't "fit" together at their border any more. It is this "not fitting together" that we see in the block artifacts