Here is one for the experts and those who have experience this type of behavior: Is it possible for hens to go broody without a rooster and try to hatch infertile eggs?
Just as hens will lay eggs whether or not a rooster is present they will also go broody without a rooster.
I'm on board with sourland - just as egg laying is hormonal, so is broodiness. Of course some breeds (and some chickens) are not very broody. I have a 7-year old Delaware who has never been broody even once. Of course, she has a little "mini broody" session for 40 minutes after laying, then hops to her feet and walks out of the nest box like nothing happened. I think she may be a freak. Where there's eggs, there's a least potential for broodiness.
Think of it this way: going broody is hormonal not environmental. I have two girls that consistently try to hatch pine shavings for about 28 days. It's pitiful.