Hypothesis 1:

Trauma-induced cruciate ligament rupture.

Reasoning:  

Trauma (a dramatic increase in shearing pressure) to the right stifle joint, 
due to falling off the bed, caused rupture to a cruciate ligament.  This rupture 
immediately compromises the stability of the stifle joint, leading to joint 
laxity.  This laxity leads to malalignment of the patellar ligament within the 
trochlear ridge, allowing the patella to luxate.  Luxation of the patella 
interferes with normal flexion of the stifle joint.