Ok. so for the future.. Add more hiding spots if your going to QT more than 2 fish at a time. Also, remember, there is a certain amount of oxygen in the water so the more fish you put in, the more oxygen they are using. This means you have to add more oxygen to the water for them to survive. Adding a powerhead to the QT and pointing it up so that it disturbs the water's surface will increase gas exchange and increase oxygen. An HOB filter will do the same. Remember also, that the meds you use can deplete the oxygen in the water along with all the fish in there using it up. They could have had a disease (like velvet or brook) on top of this, but my gut tells me they were stressed and didn't have enough oxygen to breathe.
I may have said this before, but it bears repeating. NEVER ever use prime or other water conditioners in a tank where copper has been or will be present. << This has been a public service announcement