Perhaps you would use the more common term: hens "set", dogs and people "sit"! LOL Yes it is a hormone thingy. Many breeds of chickens are natural "setters" and will lay a "clutch" of eggs (many be 15 or 50, depends upon the individual hen!) If your hens are of a "setting" breed-when the hen has completed her clutch, she will SET, eggs or no eggs in the nest. It is a change of hormones: Mother Nature says SET. Will your hens SET?, maybe--....be patient.
If you date your eggs when you collect them, save them in an egg carton for a week or 2, replacing the oldest with the newest each day or 2, (Breakfast or egg salad or deviled eggs use the eggs that are removed!) And IF your hen/hens go into the setting mode, you will have freshest eggs to place under the hen--the number she can cover safely.. Best do it at night and she will soon have them all warmed up overnight,and be ready to spend the next 21 days incubating them. Oh, she will get off the nest daily, sometimes more than once, for various length of time, and leave a large stinky poo, eat and drink, and then return to her nest! Hope you will have her in a nest where other hens cannot lay--or allow eggs to collect in another nest--OR she may move to a different nest and your first "set" will be chilled! So best prepare a private, quiet, maybe darkened area for her, with food and water right there. It also may be at ground level, so chicks will not be left high up, when they hatch and the hen hops down leaving cheeping babies, cold or worse, to fall out.
And during the 21 days, you will have their brooding place ready, with water that the chicks can't drown in, and Starter crumbles, all a great start for a lovely brood of chicks!~ (I prefer hen hatches to incubator hatching and do not want dozens of chicks at one time!) Each hen has her start and hatch dates on the calendar, so it is never a surprise--BE PREPARED. And each hen has a large cage to care for her chicks for several weeks, not always easy to merge hens with chicks, but when chicks are weaned, a LARGE area for them, with feeders at different points, so all can reach them safely, will usually keep several different hatches able to grow up without too much aggression! Just watch them, things usually settle into a happy flock! Have 2 hens setting right now, will hatch just one day apart, and as they are Bantams, with just 5 eggs each, they may be given to just one of the hens and then, just one growing pen will be necessary! Joy and pleasure! Hope you will have it! Good luck.