basically that subwoofer that is unplugged is still moving because the other subwoofer share the same box chamber as the other. If they were in two seperate like boxes inside of one box then you wouldnt see this. You might see a little movement, but not much. But since they are in the same open space, the one that is still being powered, the air being generated from that one is causing the other to move.
Since the one still works, but is unplugged, you could be causing damage to this because the subwoofer is moving against its will. when the motor has current running through it, its meant to be played, but with no current, its still and should not be played like that.
There is one instance where this would be ok which is called a passive radiator setup like mentioned above which you generally use one bigger sized subwoofer that does not even have a magnet on it. Its just going to be the basket and the soft parts (cone, surround, maybe the coil) and there are also powered subwoofers in the same chamber which are usually of a smaller size like 10s or 8s. And by those smaller subwoofers moving and creating air force, it moves that non powered subwoofer and will actually give you some of the response that you would normally see out of that woofer.
name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="
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type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>" type="application/x-shockwave-flash" width="425" height="350">Here is a perfect example of a passive radiator... the center sub is the radiator and the smaller 8s are the powered subwoofers moving the free air 15.