General reasons quail can be found dead unexpectedly:
1. They can die very quickly from infection. I had one start to act lethargic one evening, the next morning she was dead with no sign of injury. Some infections like botulism can kill within hours. They can also die from toxins or spoiled food.
2. They can break their own neck, either by flying up and hitting the roof or sticking their head through the bars of the cage, panicking and forgetting how to pull their head out. Sometimes when they hit the roof their head goes up through the bars and they hang themselves. I rescued a button quail I found hanging this way.
Anything their head fits through, they can get caught in and panic. I had two cages side by side and they'd put their heads through the bars to get at the food and water in the other cage. There was plenty of space between the bars. One of them panicked in the middle of the night with her head stuck through and broke her neck. I've since put netting on the bars to prevent another accident.
3. They only live to about 2 years old, after that they can die at any time.