camo...The alum will turn the water acidic relatively slowly. Depending on how many acre feet of water (I'm guessing about 2-2.5), the alum probably lowered your pH as much as 2 points, causing a moderate shock. Adding the Hydrated Lime, your pH would have jumped back up to the starting pH, within minutes of the H lime being applied. That is a pretty harsh shock, and easily could have been lethal.
To help avoid others causing a shock kill, alternate applying 2 50# bags of Alum, followed by 1 50# bag of Hydrated Lime sprayed in the pond, in a proportion to the total pounds and total surface area. This will help reduce the pH shock, plus leaves untreated, and pH balanced areas for fish to retreat to during application.
