This young patient fractured his hip with little or minimal trauma.   This should immediately alert the clinician to an underlying problem, suggesting a pathological fracture.

The femur is the second commonest site for a unicameral bone cyst after the humerus.   They are typically identified in early life and are more common in males.  

The fallen fragment sign is said to be pathognomonic for a simple (unicameral) bone cyst following a pathological fracture.  The reason being the cyst is filled with serosanginous fluid, which is non inhibiting to the denser bone fragment falling through it into the dependent position.