What causes fluid in the facet joints?

Accumulation of Fluid in the ankle joints is caused by the soft tissues that surrounds it but it can have many causes, it could probably be traumatic injury, gout, arthritis, infection or even edema....

Spurs and hypertrophy of the ac level causes partial of subacromial fat signal

I could find no material that indicated this problem in the case of infections in sinovial fluid's.

Instability of that particular motion segment. More than likely there is a disc bulge causing radicular pain down the left back into the buttocks and eventually down the leg.

Synovial fluid