Why could a broken clavicle affect the mobility of the scapula?

Clavicle is a strut. The clavicle is the only connection the scapula (and the rest of the arm) has to the rest of the skeleton. The clavicle acts as a strut to keep the scapula in the proper position. A shortened clavicle from a break can cause the scapula to become hunched forward, affecting its mobility.
Shoulder Girdle. Learn more about the shoulder girdle here: http://www.Theshouldercenter.Com/shoulderpain/2013/shoulder-surgery/spur-acromioplasty-impingement/.