Primary.... eggs should be all set at same time if being done by a human. Hens who gather their own clutch are basically 'storing' the eggs till they are ready to set, then once the hen parks herself on the nest all of the eggs start developing from there.
.... if a hen is setting on a clutch, you should mark her eggs if the nest is accessible to other birds. This will allow you to pull 'intruder' eggs laid by the other hens. The 'intruder eggs' (just the name my DH and I use for them) can cause problems a couple of ways... it can cause too many eggs in a nest which may result in already started eggs getting pushed out to edges and getting cold, or it can result in a staggered hatch, which may cause the hen to either abandon eggs or neglect already hatched chicks.
... if the flock isn't broody friendly, and multiple other hens routinely bother the broody then you are risking eggs being broke when the hens squabble. The broody will try to defend her space, and can even be injured when doing so and eggs can be broken. When this happens then alternate nesting/segregation measures should be taken.
.... broken eggs (from issue above) are not only a problem because you lost the broken egg, but you also risk contamination of the other eggs and it may result in a secondary loss of 2 or 3 more.
... overwhelming broodies with too many eggs. Some large fowl hens can cover 18+ eggs, but maxxing out a broody for coverage increases risk of broken eggs and eggs which may have development problems because they are too far out from under the center of the broody for too long at a time. The broody will shuffle eggs in an attempt to keep all of them warm, but again, having 18 or 20 eggs can make it problematic for her. Why cause the problem if simply reducing the egg # by 3 or 4 eggs can make it easier on her? Extremely large clutches of chicks can also be problematic for the hen to keep warm after they are 2 or 3 weeks old if the weather isn't cooperative.
