Well, it could be that they are trying to keep her eggs from incubation, which some hens will do. But if the chicks are doing this too, I would suspect "monkey see, monkey do". Your older hens could be suffering from protein deficiency and teaching the chicks a very bad habbit. I am not sure what layer crumble you have them on or what kind of birds you have but from our experience, August is a time for molting, which means low protein levels. Cracked corn does nothing for protein, I would try supplementing their diet first. There are several ways i.e. scrambled eggs, meat products, etc. What works for one may not be the best for you so I would research it or PM me for how we help our girls. I would also cut out corn as it is just a filler, but that is just me. I would also try a higher percentage protein feed, like what is used for game breeds or turkeys, just until they get back to normal. Are your girls molting at all?