The mouse trap thing that comes down to hit the mouse may have hit your lil duckling in the head which may have caused serious brain injury. We had mice here when we moved in and my dh set up some traps. One mouse set off the trap and from the way it was acting I am pretty sure it got hit in the head. My Dachshund got it. before i could get it outside. Poor thing def was not right.
I am so sorry but it is amazing how far ducklings can jump so having a brooder with some kind of top is best. I use old window screens and place a couple 2X4 on top to keep them tight.