For future reference, if the fish have been in the bag for much time, do not open the bag and start adding your water to the bag water, just remove the fish to the isolation tank. The bag water has had the fish respire carbon dioxide into the water making it more acid, and they release ammonia into the bag, which at low pH is not toxic. Once the bag is opened, some of the carbon dioxide will start to leave the water and the pH will start to climb making the ammonia more toxic. Adding your water accelerates the pH shift and accelerates the toxic levels of the ammonia. Getting the fish out with the least time that the bag has to be open is best.
Some dealers promote the adding of your water to the bag water, but the only way that works is to have the fish in the bag a very short time and the bag essentially untied or not oxygenated as done by the good dealers. I have never experienced a problem with moving the fish immediately following taking them to shows where they had 4 or 5 hours in the bag or longer. They are pretty tough when it comes to moving into the pond. The pH shift isn't a problem, the hardness shift has never been a problem, but the temperature shift should be minimized by floating the bag in the shade as long as possible. I have never experienced your problem so not sure of the cause.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard