One day I finished cooking a beef stock. When I poured the stock into a glass bowl and added some vinegar, a cloudy precipitate formed and remained suspended in the stock, leaving it somewhat cloudy.
Chemist chefs, what happened?
Gelatin is pH sensitive. Acidic ingredients can make gelatin become cloudy. Stock is naturally rich in gelatin, so a similar reaction is probably happening in your stock. Acids cause many proteins to coagulate, in some cheeses for instance, so the acid is probably causing the suspended proteins and gelatin in your stock, which are normally not visible, to coagulate into what appears to be cloudiness.