Most ponds and lakes become stratified when the warm water on top separates from the cooler water on the bottom. The deeper water becomes toxic because it cannot obtain oxygen. When a cold front hits, it cools the water in the top stratum. This causes rapid mixing, which brings the toxic water to the surface. The toxins can kill the zooplankton and fish, taking up even more oxygen. Fish stressed by the change in pH and toxins are now forced to struggle with low oxygen. Depending on the severity of the turnover, fish may start dying.
Usually, the oldest fish die first.