From the time they are chicks until they are fully matured, they are doing one long, slow molt. They need to grow feathers that go with their newer, larger size and they have to get rid of their old, smaller feathers before they can grow the new ones.
Young chickens do a lot of sparing as they mature. Even established flocks have their little squabbles from time to time. Adult hens get broody or molt and that can cause a little more fussing, too.