Notice the arrow pointing at the low back. This irritates the spinal ligaments and can compress nerves as well. The slouched posture has another drawback. Once you anchor your body against the chairback, you loose freedom of movement of the torso. This forces all movement to be translated to the upper extremities, leading them prone to repetitive strain injuries such as tendonitis.
Freedom of movement is imperative for musical expression.
It would be quite difficult for a pianist to play expressively with his back resting against a seat. The same goes for any seated instrument. Cello players, for example, must use upper body movement to translate the bow across the strings. A slouched posture will force all bow movement to the shoulders, arms, and wrists, thus leading to strain in these structures. Just as a baseball pitcher uses his entire body to propel the baseball forward, so does a musician use her entire body to translate movement into her instrument to produce beautiful musical tones.