The reason they drain down lakes in the winter is to help improve the health of the water.
The water in lakes are drained down during winter months to help freeze out aquatic weeds so that the water doesn't become affected by those weeds during the summer.
During the fall and winter months lake activity is down so that's a good time to lower the water levels of the lake to kill off those bad weeds in the lake water.
The draw downs of the lake water in winter months can dry and freeze the offending plants helping to control invasive species.
Not all lakes are drawn down every winter but some lakes are and some lakes are only drawn down two years in a row every 8 to 10 years.