As Brain eluded to I believe you had a turnover with anoxic water coming up to the surface. All the leaves you have on the bottom have probably created an ideal environment for oxygen depeletion on and near the bottom.

Have you ever tried preventing stratification with diffusers? Sounds like you haven't and if you did you probably wouldn't have the fish kill you have now. Maybe an investment for the future.

Sorry about your misfortune.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.