Why might someone be succeptible to knee cap dislocations/subluxations?

Anatomy: The trochea is the part of the tibia (leg bone) that is the groove that the patella sits in and slides up and down. If that is shallow for whatever reason (your parents, bad luck) then its just physically easier for it to pop out of place. With a shallow groove the muscles and ligaments might not be able to keep it stable. Strong quad muscles will help.

Anatomy: The anatomy of a persons knee can lead to knee cap (patellar) dislocations. Patellar stability is dependent on: bony constraint of the femoral trochlea; mpfl, medial retinaculum, vastus medialis oblique. Any of these may be disrupted or dysplastic in patients with patellar instability.

Bony architecture.: We are all born with slightly varying bony architecture. Some people are born with kneecaps that are prone to dislocation, because the tissues that keep the kneecap in place, are affected by poor alignment of the kneecap.