The waist appareance ‘‘Pale Rider’s Eternal Girdle’’ has physics bugs when the character is moving, causing the waist to still move in unrealistic ways even when you are no longer moving.
This issue has existed since patch 11.0.5 when it was implemented.
Edit: I added a video that proves the bug
https://www.youtube.com/watch?v=YXAiinH0vX0
If you are a DK and you also want this bug fixed I recommend doing the same thing I did, submit a bug report in-game and show support for this post so it can be given the attention it deserves, this DK transmog is one of the best we’ve had and it would be terrible if it had a bug like this that could easily be fixed.