Why does the tuna Casserole turn sour

The same logic applies. The smaller the temperature difference between uncooked and cooked, the easier it will be to get an even amount of cooking all the way through. Especially with a fast method of cooking like broiling.

As far as I know, raw tuna is not usually salted and/or pre-treated.

What may be happening is a case of perceived salinity. For example, there are numerous studies that suggest foods with certain odors can be perceived as much saltier than they actually are. If you have access to that second article (it is copyrighted and behind a paywall, so I can't post it here), it has a great figure on page 3 depicting the perceived sourness, bitterness, sweetness, and saltiness of a number of common foods, including tuna. The aroma of plain tuna alone ranked higher than soy sauce on the saltiness scale, for example. Tuna is relatively high in fat, and other studies suggest that high fat foods can also increase perceived saltiness. Perhaps you were using a fattier part of the fish?

Finally, there is evidence that dehydration increases the saltiness of tuna (which actually seems somewhat intuitive), especially if extra salt is added before cooking. Was your tuna previously frozen? Freezing—especially if done improperly and/or if there are multiple freeze/thaw cycles—can act like dehydration.