Like, lost-ALL-of-them-looks-like-they-have-cleavage, or didja pick them up and go"OH MY WORD!" My Cochin lose some breast feathers- a strip on either side of the breatbone- when they start to lay- I dunno why.
I noticed that some of mine started plucking chest feathers when they were just THINKing about being broody. Keep in mind that if we weren't stealing their eggs, chickens are thinking about being broody - laying a clutch TO set - a week or two before they decide to set and hatch. So they could just be getting ready and plucking feathers to keep the eggs comfy while they wait.