I'm quite sure it's an intended behavior to give the player a visual feedback of the less stability you have in those stances, which are not natural.
Infact if you look closer, you can notice the step lean left, that changes weapon shoulder too, shakes more than the others.

/downvoted for now, will upvote if a dev or a mod confirms it's a bug!

The insta-shake upon adjusting stance is a feature, but i think it looks quite unnatural.
imo, something more gradual, subtle and not so immediate would be better.

reduced the shake on internal dev version - should get into pubic with next update (that may take some time because Beta)