I really believe sometimes the girls start hearing peeping and if any are having "mama" thoughts it can put them over the edge.Tamar: "I want chicks, too!"
Not always but on more than 1 occasion I've had 1 girl hatch babies and within a week or so other girls start clucking, puffing up, etc.
Coincidence? Maybe.
I let 3 sit one summer. Lots of fun but that was a busy summer for me!
And all the boys.
I like to encourage broodyness in my flock. I love to watch a hen with chicks. Sometimes I have to force myself to set limits. Babies are so sweet!