I think it happened due to (1) warmer temperatures - warmer water holds less oxygen and oxygen is consumed at a higher rate by organisms, (2)several days of cloudy weather - no sunlight, no photosynthesis, no oxygen production, and (3)no wind - no windmill aeration and no surface disturbance or diffusion of oxygen at the surface. The only thing that could prevent this from happening again is a reliable electric air compressor.
