Since one of the latest updates, people's lower body IK no longer turns off while they're sitting in a seat. They will continue to shuffle their legs and hips around as they move their head. When the station moves, their legs will also do footsteps as if they're standing.
The person sitting down can't see this happening themselves. For themselves, they will appear to sit in the chair normally. Everyone else sees them still move their legs and hips.
This happens on all chairs, both in worlds and on avatars.