Poor choice of encoder and or incorrect settings used in the encoding. Too low a bitrate for a motion scenes is probably the cause.
What size are the files you downloaded compared to their running time? What format are they in, and what codec was used in making them? Did you convert the files to dvd-video, or did you simply burn them to disks in their downloaded format?
Use a program called MediaInfo to examine the downloaded file and post your results here. I find the Tree view within MediaInfo to be the most informative.