Trying to commandeer the nest?

Okay, I got fertilized eggs the other day, and put them under my broody hen, Juanita. Today, there was a dispute, because one of my other hens got into the next box while Juanita was out gobbling some grain, and decided they were HER eggs. Then I went out a few minutes later, and they were BOTH in there trying to brood the eggs. I removed Sophie (the latecomer) and made the nest box inaccessible with the temporary cage. Now Sophie is in distress and acting like her chicks were taken or something.

Is there something about fertilized eggs that's different? Sophie wasn't like this before we got the fertilized eggs, and these have only been incubating for three days, now.