Been reading through this:
Somatogravic and Somatogyral Illusions
"Somatogravic and Somatogyral illusions are the two most common forms of vestibular or 'false sensation illusion' which may be encountered when no clear horizon is present and flying wholly or partly by visual external reference is attempted."
skybraryDOTaero/index.php/Somatogravic_and_Somatogyral_Illusions (replace 'DOT' with '.')
I have experienced this while driving at night through light to moderate snowfall. Any visual reference to a fixed point is masked by weather, your brain tells you that YOU are the fixed point which creates the odd, unnerving sensation that you are standing still. You look at your tach and speedo, all is normal. I would imagine that as a pilot, with a third axis to deal with, the sensation is several times worse...