I honestly don't know what could've happened. I followed the same process with my water that I usually do. I use a digital thermometer and a heater to bring the water to the same temperature as the tank and dripped the new water in with airline tubbing.
I lost a spawn not too long ago the same way because there was a problem with our local water. I found out too late that the city was advicing to boil the water.
It was too late at night for me to check water parameters, I just did the water change and thankfully it was enough to get them back on track.