When you say thrashing do you mean just moving around and kicking? Or something more like convulsing? If you assist the hatch sometimes they will continue to move around as if they are still trying to unzip the eggs until they realize that they are free. Especially if you have them in a small...
I haven't had it happen with broody ducks since I use an incubator, but I have had feisty ducklings kick around an egg chipping off most of the eggshell before the membrane was unzipped. If it's day 28 I start to unzip the egg in small amounts (1/2 inch at a time) every 2 hours. This keeps the...