I experienced the same problem with a CPU going crazy and my all system drastically slowed down. I had to take patience to kill the process and retrieve my system stable.
I noticed that it happened with a thunar window opened in the background. Surprisingly, the displayed directory in thunar was not the one where the video was downloading, but a parent of it. That seems to mean that it concerns not only current displayed directory, but also recently opened directories.
I experienced the same problem with a CPU going crazy and my all system drastically slowed down. I had to take patience to kill the process and retrieve my system stable.
I noticed that it happened with a thunar window opened in the background. Surprisingly, the displayed directory in thunar was not the one where the video was downloading, but a parent of it. That seems to mean that it concerns not only current displayed directory, but also recently opened directories.