Final answer:
A Hill-Sachs deformity is caused by humeral head compression against the glenoid cavity of the scapula during a shoulder dislocation.
Step-by-step explanation:
The primary cause of a Hill-Sachs deformity is humeral head compression. During a shoulder dislocation, the humeral head is forced against the glenoid cavity of the scapula, causing a compression injury. The repetitive compression can result in a defect or groove on the head of the humerus, known as a Hill-Sachs deformity.