How or why are two different questions. How is something a doc would have to explain. Then theres WHY.
My guess would be the effects of adrenaline on the organs. Most likely its a combination of adrenaline and a low metabolism. As in not usable adrenaline.
Its kinda like nitrogen injection in cars, push a button and you get an instant add boost of horsepower, use it to long, and your engine blows up. Adrenaline works that way with the body.
In the case of monitors, stress causes systems to break down, casual stress, simple poor husbandry causes the immue system to be compromised and monitors get mouth rot, colds, sores, RI, etc. All these things do not effect healthy monitors. Also with poor husbandry, monitors experience tumors, which again do not occur with healthy monitors, these tumors are mostly hmmmmmm puss from an systemic infection. All this from physical stress.
Holding animals that do not want to be held causes them to produce adrenaline. Their heart races, etc. Only theres nothing they can do.
Such enviornmental stresses as low basking temps, cause organs to function at the wrong temps. They then fail over time.
Monitors also suffer from mental stress, being confined in places with nothing they recognize. Remember, you know the difference between natural materials and manifactured materials. You do because you were taught. If you were never taught was carpet was, and you had never seen it, read about it, or hear of it. Your first reaction would be of fear. Monitors are the same, they fear what they do not know. So while you think paper and carpet and such things are nice and easy to clean, to the monitor the are indeed alien.
Monitors also get stress from adverse social conditions. Again, this stress would be like putting you in a prison cell to like with the biggest meanest killer in a prison. He may or may not want to kill you, but do not know that. So it would cause you lots of stress.
I could go on and on and I usually do, but I hope you get the point, if you were confined in adverse conditions you too would breakdown and die.
What is an indication of organ failure. Very simply put, its when a healthy looking monitor dies. They can have good body weight, normal bone structure, they just fall over dead. Well you should understand, something has to have a complete failure to cause death.
When healthy newly acquired monitors die, it can be traced back to over-medication. They have been treated for various things too many times and in most cases, improperly(not by a vet)(by a petshop employee or an importers employee)
Normally a monitor should look dead to die. It should be all shrivelled up and bones sticking out, etc. Not look healthy. Again, common sense.
My guess is, most healthy animals die from lead poisoning(shot) the rest die from organ failure. Of course this is only a piece of the iceberg, so to speak. Cheers