You know they have a mind of their own and do whatever they want to. To prove that point,I have 2 white leghorn hens setting at present. One is setting for the second time because the second hen decided she wanted to set on the first one's eggs just a few days before hatch day and ended up squashing all of the eggs. I didn't have the heart to break her up,so I re-set her. I'm watching her close and making sure she gets off to eat and drink. It amazes me at how perfect this process is. I think the first eggs were squashed because the shells were becoming more brittle(in advance for the hatch), and the second hen was just too heavy. By the time a broody hen reaches hatch day,she has lost alot of weight and the eggs are protected. You just can't think these things up and no way could it just "happen" by chance.