Why is the collarbone so easy to break?
The collarbone, the clavicle, is one of the easiest bones to break because of its role and its location. While it is possible to break it by a direct hit, it also can be broken by a transfer of shock. This means, if you fall hard on your shoulder or on an outstretched hand, the shock of the impact could travel along to the collarbone, breaking it.
Charles T. Mehlman, DO, MPH, et al. Operative Treatment of Completely DIsplaced Clavicle Shaft Fractures in Children. In Journal of Pediatric Orthopedics. December 2009. Vol. 29. No. 8. Pp. 851 to 855.