The most common underlying cause of hip and groin pain is weakness and lack of control of the gluteal muscles resulting in a combination of excessive forward and lateral tilting of the pelvis when running and walking. Since the hip joint socket is part of the pelvis any alteration in position and motion of the pelvis while the weight is on the leg during walking and running directly increases stress to the hip joint, especially when you consider that impact forces in the hip joint when running approach 3-4 times body weight. The benefits of losing weight can therefore be seen. If you lose a kilo of weight you decrease the forces through the hip and knee by approximately 3kg per step.
Fractures and cartilage or labral tears often result from direct trauma involving rotation. Stress fractures can also be seen around the hip in long distance runners.
There are congenital conditions of the hip where the socket is shallow or bone lumps develop behind the ball section on the femur. These can cause the bone of the femur to impinge against the bone of the pelvis leading to restricted movement and pain. Relatively new techniques with CT scanning show these anatomical variations amazingly well.