If the filter was not keeping up with the load, the ammonia level would have been up, as well as the nitrite level. With a strong storm, heavy amounts of acid rain could have caused a pH crash, which would kill the filter, and potentially the fish. It could also have caused the proteins in the pond to foam up, but I would think it could also break the foam so it would not be as heavy as it was. I have never seen a pH crash that killed everything as fast as you describe. Algae will consume oxygen during the night, putting carbon dioxide in the pond through the reverse of photosynthesis, and the high carbon dioxide would have two effects, pH lowering/crash, and problems with the fish being able to expel/release the carbon dioxide from the blood, causing some form of suffocation, but that is typically slow and affects the largest fish first, lots of piping by all survivors.
The test results you posted earlier don't lend themselves to any of the above.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard