An uke (passive or not) can regain his balance because it's a totally natural thing to attempt to do.
I disagree. It is easy to experience by yourself. Practice unbalancing with first time beginner, and put him on the tatami. Nobody will regain 'naturally' his balance. They are collapsing and crashing on the mat.
In fact there is no universal definition of 'doing natural movement'. So it is useless to use it here.