Patellar instability - subluxation or dislocation - can be congenital or acquired. A congenital condition is a condition with which someone is born.
An acquired condition is a condition that comes on due to injury or incorrect surgery.
Congenital patellar instability
Hypermobility is something one is born with, and is usually genetically inherited. The person with 'rubber joints' or 'double joints' usually does not need a doctor to tell them that their ligaments are lax, but it may take a while to determine which of the various hypermobililty syndromes (eg Ehlers Danlos Syndrome, Marfan syndrome)is causing the hypermobility.
People born with generalised ligament laxity will be used to doing party tricks to amuse their friends, like bending their thumb to touch their forearm or bending their fingers backwards. But unfortunately - although great for party tricks - excessive joint mobility is frequently associated with knee problems, and in particular recurrent dislocation of the patella.
Congenital Patellofemoral Dysplasia
Dysplasia refers to a different problem - one in which the patella and/or its underlying groove is an abnormal shape - in which case the patella is not properly contained within the groove when the knee is bent and may dislocate.
Acquired patellar instability
Acquired patellar instability is where patellar subluxation or dislocation is not due to a condition one is born with, but is due to a problem that occurred later, possibly due to injury or incorrect surgery, or even just to weakness of the muscles associated with the patella.