There is always a cause of death, but not all of them can be detected in a quarantine tank.
Even once you introduce the new fish I to the main tank it doesn't mean you are safe. The idea behind quarantine is so that 1. You can watch for weird behavior or I'd some physical illness. If the fish colors up, eat great, displays normal behavior for that species then the fish is likely okay. Assuming this has been the case for 3-4 weeks.
2. We also quarantine to ensure that the fish is in IDEAL condition healthy wise and lose stress. This is because there ARE pathogens in each aquarium. All it takes is a stressed fish to enter it and the weakened fish is now sick. But if that fish just came from your other tank which not stressed and well fed etc etc. Then the chances of the fish getting sick in the new tank is far less likely.
So we want to ensure the new arrival is healthy but also in good enough condition to fight off anything that is in the main tank.
If the quarantined tank is poorly done, the fish inside that tank will surely be stressed their entire stay. This could been months, but that doesn't mean that it's healthy and ready to go into the main tank. That's why is so critical to setup QT tanks with ideal habitat for that species. Not like a hospital tank where you want it bare as can be cause you know the fish is sick.
QT tanks is to isolate till the condition of said fish is 100% good to go.
Hope this helps clear things up.