http://blogs.windsorstar.com/2012/09/07/water-samples-support-fish-die-off-from-natural-causes-moe-says/
the lake "flipped" and the resulting fast temperature change and oxygen depletion killed them off. "flips" happen in the fall (usually) when the coldwater layer at the bottom of the lake "flips", rises, and displaces the warmwater layer that has developed all summer. It can be sudden and often wreaks havoc on oxygen levels at depths where fish are located at that moment in time which can kill, let alone the sudden temperature change.
Sad natural phenomenon, thousands of perch, bass, drum were killed. This can devastate a smaller body of water and set the fishery back years. Now what will happen is that scavenger birds will eat the rotting flesh and die of botulism. Ugly chain of events.