I am also very sorry for the loss of Sal
My first thought was he may have had heart problems and something set him off to the point he had heart failure.
The point about current running thru the ground is also excellent and again, if Sal did have heart issues, that could have also triggered heart failure.
Sal wasn't too young. I am forum acquainted with a lady who lost her APHA to serious heart problems when he was under ten years old. I want to say he wasn't much more than five but it's been several years back and I can't remember. He was the most gorgeous Paint I have ever laid eyes on and as gentle as he was handsome
Interesting and mysterious the previous owner hung up on you
Since you have taken every scientific step known to mankind, I believe I would step out of the realm of what is normal and go digging.
Electric current running thru the ground periodically is a good scientific first step. It does not have to be your electric current either. How are far are you from power sub-stations? Are there any of those giant power grid cables running nearby to where anything might have an electric leak that is not enough to affect a human but it would affect animals?
Does anyone near you have pigs? Pigs are extremely sensitive to electric. They wear rings in their noses to keep them from digging because a jolt from an equine fence charger can kill them.
If someone with pigs periodically might experience losing some, leaking electric current is the answer.
Then there's always the realm of things we don't understand. How long did the previous owner live in your home? Any chance you might be able to track down the owners before them or relatives?
How about long-time neighbors that would know not only the history but the lore of your area?
Some THING is triggering your animals and it's real to be affecting all the species you own
You sure have my curiosity piqued. Please update us, if you find out any little thing