It generally means that she has dislocated her stifle joint. The stifle joint is on the hind leg just below the flank - it's a relatively flat joint and can in some animals wear to the point where the ball of the joint slides off of the plate - when that happens in can get stuck and the animal will appear as if it's hind leg is stuck out behind it. With cows, I think this is most often caused by the animal slipping and falling on a slippery surface.
