Wow, that is BIG! As lucky as they are to us - it must have hurt the poor thing. And now you don't have space in the incubator to prove or disprove our theories...
I think I remember reading somewhere that it might possibly have to do with the feed, if the hen or duck had too many pellets (if it happens a lot, the odd one is probably a normal oddity). A condition called egg binding (problems passing the egg) can also be caused by overfeeding on pellets. One of our girls had it twice last year and the poor thing was in so much pain, she could hardly walk. Reading up on it it said it is most likely a recurring condition and best to put her down! I then reduced the pellets and replaced the amount with extra wheat and barley or mixed corn and she's been fine since.