Turnover is a physical function that is caused by the change in water density as it cools or heats. Water is strange stuff, it is at its maximun density at 4 deg C (about 39 F). Ice is colder but less dense, that is why it floats.
In the fall, as it cools on top, the water gets denser and eventually "turns over" or falls to the bottom. This displaces the less dense water and stirs things up.
In the spring, as the ice melts or surface water gets up to 39, it gets more dense and settles to the bottom.
What you see at other times of the year is not turnover but turbidity caused by wind, lake gasses, or alge blooms.