I think turnover being associated with bad fishing is because there is a major change in their habitat. The water temperature changes, the wate clarity changes, and suddenly the oxygen poor areas in deep water are suddenly available to them to spread out. Once the turnover has been over for a time, then the classic fall patterns start and great fishing happens.
Turnover happens in fall. Water is an interesting thing. It is its most dense right before it freezes.
Turnover is different depending on lattitude, elevation and depth, but in Wyoming it generally occurs twice a year when the water is all the same density (at 39 degrees). This usually occurs a few weeks after ice out in the spring and again in fall when the water reaches a uniform temprature and mix with wind or water currents.