Assuming since you mentioned pond at about 100 gallons...down from 130 due to evaporation, that it was warm out temperature wise prior to and during the thunderstorm and rain periods.
Wierd things can happen to your oxygen levels when this happens. I know as I witnessed gasping behavior one year in my smaller pond just prior to a summer storm. Fortunately I was out back and saw it....immediately cranked on the waterfall which was off. Within 15 minutes all was normal. If I had not seen them gasping and had stayed in during the storm, they would have expired. It appears your oxygen levels (5-8ppm) were on the low side anyway. In my opinion....lack of oxygen due to barometric pressure or whatever resulting in fish die off.
Sorry for your loss.
About the time you get old enough to realize your father was a smart man, you have a teenager telling you just how stupid you are. :D