Eddies occur where there is water flowing in one main direction. Sooner or later there will be an obstruction near the edge of the current and some of the water can't continue downstream so gets redirected upstream, it can't go in a contrary direction for ever and is eventually caught up again by the current and heads back down again. Thus the contrary flow has become circular, (actually it's usually oval.) Try looking where a log has fallen into the water at the edge of the stream.