There are tons of people smarter than me, but I have seen some similar scenarios play out, incliuding on my Cape Buffalo bull. It has led me to frequently say that not all double lung hits are created equal.
As Terry said, most people would take this shot everytime if they can make it, and it is still a little hard to believe that the hog lasted as long as it did, given the blood trail.
Now, there are alot of misconceptions about what happens when a broadhead penetrates the lungs. Contrary to what many think, the lungs really don't resemble balloons. They don't instantly deflate. They are composed of very small air sacs, called alveoli, that communicate with the airways, as well as blood vessels, and lymphatics for fighting infection. The further along the perphery of the lungs you are, the smaller that the airways are that supply the alveoli, and the less the lung is likely to deflate initially. The blood vessels also are smaller and there is correspondingly less bleeding. Now, as time goes on the bleeding gets into some of the other airways and blocks them and causes the lungs to collapse and reduced trasnsport of oxygen to the blood, but you can see how this can lead to longer trails, both in terms of distance and time to expiration. If the hit is too small, especially if only one lung is hit, the animal can actually live.
Now, I know this may be contrary to what some people think, but people can live without one lung as well, although that is usually after a surgeon controls the bleeding. It is important to realize that people have lung biopses and partial resections everyday, and still survive. I don't want to infer that these are the same situation as putting an arrow through an animal, but there are some parallels.