That's a little young, but chicks go through two molts before they reach adult size. I'd really expect your first molt in another 2 to 3 weeks, not this early. It may be nothing, but the first thing I'd do is check for mites and lice. If it is roost mites, those only come out in the dark so you need to take a flashlight out after dark and check around the vent area. They will run from light so try to keep it fairly dark when you are doing this.
It's possible that the other chicks are pecking the feathers, but they usually eat them when they peck them. Of course, with chickens what they usually do and what yours actually do are often different things.
It may have something to do with stress, maybe from heat or something else.
I'd check for mites and lice but otherwise really would not over-worry about it. It's an indication that something might be wrong, not a guarantee that something actually is.
Good luck!