I believe it's related to this. Try leaning into the bore fast then pulling your head out, fast. Many people lose their balance temporarily. Something to do with inducing currents in the fluid in the inner ear I believe. A smaller version of the effect causes the sensation that you are moving when you lie still in the bore.

Edit: can't get the link easily as am on iPad, but google "MRI eddy currents" and watch a slab of aluminium fall