Chicken behavior is utterly fascinating. If you spend time with your chickens, watching them interact and react to things, you will come to understand what their different vocalizations mean.
The first step to understanding why they are noisy is to observe them when they're doing it. Can you see what may have set them off? Are they outside or in the coop when they do it? Where they are when they are vocalizing will tell you a lot.
If they are outside roaming when they suddenly become very noisy, it may be they're issuing an alarm over seeing an animal that concerns them. If they're inside the coop when they're going off, it's most likely they and other hens are in egg laying mode. It's not uncommon for several hens to join in issuing the egg song vocalization when one is in the nest trying to lay an egg, or right after one has laid an egg.
Those are the two main reasons for noisy vocalizations. It's very nearly impossible to stop them from doing either one. If you have ordinances against noisy chickens and you have mean neighbors who object to the noise, you're SOL, in my opinion.