We had ducks, and sometimes for no obvious reason, it can happen that they have a stroke or epileptic episode.
It can be genetic, or from a time of developing in the egg.
And the duck can go on for years without anything being visible, and out of the blue, they cramp up, flap around wildly, and die.
A blood vessel in the brain ruptured, some unknown factor causing them to cramp...and that sure will terrify the other birds who witness it.
You might wanna to clean up the duck house, get new bedding in, make it extra comfy for the birds and offer them treats.
If they don't want to go in anymore for a time, thats normal.
But changing their home interior up might help to get over the bad memories.
And if you got a night light to put inside so they won't spook in total darkness, it might help them to accept their home again.
For my old duck it was a flash light of a camera that made her cramp up and die.
Something in the brain just misfired, caused by the sudden bright flash and caused her to die. Bodies are strange that way.
Good luck to you and the duckies