I have several questions,

1. What was the tempeture of the tank?
2. What was its diet?
3. What was the age?
4. Where they wild caught animals?
5. What the substrate?(bedding)

Often salamander loss depends on some of these factors. But sometimes they do pick up harmful bacteria from prey items that is not oftem detectible. But as you mentioned, stress is a common cause of death among caudates.