Egg eaters always leave a mess in the nests, they can't get it all....soaked slimy bedding will usually tell the tale.
Adding new birds always causes some stress, sometimes a lot of stress especially if space is tight. Newly laying birds get rambunctious in the nest arranging the bedding, so that could also be a cause of cracked eggs.
You are probably right, just stressed out new birds who have been demoted in pecking order. That and the new younger birds rearranging the nest box expelling the needed amount of hay for a soft landing spot for the eggs.
The nest boxes are wood bottoms with hay for bedding. In hindsight I should have installed carpet onto the wood before adding hay. Short of that I keep adding more hay.