If you have a lot of water run into the pond during a storm, and out during a heavy rain, the cold rain water will displace the hot surface water rapidly or insert itself in the layer that the fish normally are living. This new water can have drastically different chemistry then your normal pond water, which will also stress your fish.
I have seen the evidence twice this summer when heavy rains created a very muddy inflow. Looking at the pond right after the storm, the water appeared clear until I noticed a very well-defined muddly water layer on the bottom 5 feet down. A day later all of the water was muddy once it mixed in the wind.
I would suspect that if you have a rain and hail storm, that inflow water can be cold enough to drive the anaerobic layer up from the bottom and do some crazy things to the bacteria down there by inserting oxygen. Who knows what, but the fish probably don't enjoy that.
