Dislocating knee cap. why is it so mobile!?

Many reasons: Recurring patella dislocations can be related to an overly shallow groove in your distal femur, to malalignment of your knee, or to previous damage to the patellofemoral ligament that helps stabilize the patella. Ask your orthopaedic surgeon.

Many possibility.: Reasons may include loose or damaged surrounding soft tissue or bony structures.

Hypermobility: The knee cap is highly mobile because there is very little bony constraint. The bone is imbedded in the quad tendon and helps as a lever arm to produce a strong force during knee extension. When the knee is straight and the quad is relaxed, there are very little soft tissue static restraints that hold it in place. There are also anatomic abnormalities that can predispose people to dislocations.