Turnover happens every year so its not really a phenomenon. Basically during the summer the cooler more oxygenated water is lower. The layer of water that has the best temperature and best oxygen (to put things simply) is called the thermocline.

The thermocline is where many fish suspend during the summer.

When turnover happens in the fall, it basically reverses, the lake well... "turns over" the colder water is on top the warmer on bottom. This is because water is less dense the colder it gets (why ice floats) combined with colder weather cooling the top quicker etc.

The lake your speaking of, I would HIGHLY doubt it has turned over this early.

How do you adjust?

I'd go read the "fall transition" thread for some great ideas.