Hypothesis 1: Trauma

The fall from the bed could have resulted in injury to the distal femur.  The fracture might not have stabilized properly so when remodeling occurred the femur might have been offset.  This could lead to weakened support for the patella and could also disrupt the stifle joint.  The weakened support due to improper fracture healing could result in luxation of the patella.   This would explain both the initial and recurring lameness.  The initial lameness could have been due to distal femur fracture.  This would resolve as the fracture started to repair itself.   The lameness that occurred six months later then would be a result of the luxated patella or disrupted  stifle joint.  
The fall from the bed could have also resulted in direct trauma to the stifle/patella such as a partial tear of the ligaments supporting the patella and/or stifle joint .  This could have led to the initial rear limb lameness.  Through rough play with the other dog in the house the dog could have further damaged the stifle joint.   This would explain the recurrence of the mild lameness.