Sunken chest in some cases is genetic, a birth defect, however, I believe in many cases it's actually not and that like cfd it's common to develop this later in life and that there's varying degrees of sunkeness. With bad posture the way your muscles are engaged and how you breathe change and this will effect the ribcage. A slouching posture and more forward craning head disengages the abdominals, a head position that is further back engages the abs and pulls down the ribs and pulls out the middle of the ribcage. With less engaged abs a persons breathing pattern changes from diaphragmatic to belly breathing, this is exacerbated by slouching posture. When the abs are properly engaged a person breathes in such a way where the ribcage is expanded outwards, the ribs aren't flared and this outward expansion is mainly anterior-posterior, people with a sunken chest lack expansion of the ribcage anterior-posteriorly.