Funny because my free ranging hens just recently started to lay their eggs on my backporch chairs. So now I can actually hear them right after they lay their eggs. My kids (twins aged 19) asked me just yesterday why do they cluck so loud after they lay an egg and I said, "because their so proud and want to yell it from the rooftops!"
If their is a scientific reason I would love to know it but if it is because their in pain from "laboring" an egg, well, then, I will just go with the proud story! hahaha