When I read your story I wanted someone who knew a little more than me to answer it. Then I forgot about it.
You say you have a lot of trees which drop leaves that use oxygen to decompose. This leaves the bottom layers of water low in oxygen. A lot of rain can stir it up enough to cause a fish kill.
Also, algae creates oxygen with sunlight and uses it during low light hours. All the clouds with the hurricanes could have dropped your oxygen levels.
The mosquito fish were probably swimming all over the place looking for o2.
One more thing. If it was due to oxygen, it killed the largest fish first.