Hypothesis 2
Congenital/Genetic
Small animal may be predisposed to congenital malformations of joint.  This could 
manifest as a joint shallowness, muscle development problem, a misalignment of 
structures,etc. leading to possible luxation.  Ligament, tendon, cruciate problems 
congenital.  The fact that this dog is overweight only exacerbates congenital problems 
that already exist.

Supplement
Often small animals are prone to obesity causing increased stress on joints and ligaments 
and predisposition to degenerative joint diseases.  Medial luxation is common in toy 
breeds--4 to 8 more times likely in a pomeranian.

Luxated patella may be considered genetic by some, but itŐs actually the conformation 
that is inherited and is a predisposing factor to the luxation problem.  It may not become 
evident until after secondary trauma.

Other congenital problems (besides patellar) that need to be ruled out 
include OCD, which can occur in one joint but usually is limited to 
large and giant breed dogs, and usually occur in more than one joint.  
Hip dysplasia can happen in just one hip, but again, is usually more 
prevalent in large breed dogs.  Avascular necrosis of the femoral head 
(Legge Perthes)  is common in small breeds and usually confined in one 
hip.  Osteoarthritis secondaay to meniscal or ligamnet damage due to 
abnormal cconformation of the joints. 

So, we have a small breed dog that was potentially born with conformational 
abnormalities in the stifle area, in addition to potential muscular/structural abnormalities. 
The typical  pathophysiology of patellar luxation stems from this as well as subsequent 
environmental factors, including injury and obesity that put abnormal forces on the joint, 
causing clinical signs.