Could be some some bone morph merging that went wrong? Sometimes pose switching get a bit wonky, a "reset pose" ocasionally resolves it if it's not a problem with non-pose morphs.
On a side note, please write a better title next time, for example:
- Why do characters sometimes shrug?
Your current title is more suited to the body of the thread as it's very long, a title should be short and to the point as much as possible.