It looks like the deer died from bleeding lung(s) and suffocation from the combined effects of the pneumothorax and blood in the lung(s).
I would very much like to have been present when it was gutted. My tests on TTSXs have me believing that the bullet should have expanded in pretty ad copy fashion at 2100 FPS. As high as that hit was, full expansion should have ruptured the descending aorta which would have put the deer down in 50 yards or so. I have deliberately shot deer at roughly the same height to prove that, but at substantially higher velocity, and that is exactly what happens. Rupture or even just a good nick to the descending aorta and they'll bleed out at about the same speed as a heart hit deer.
Maybe a little further back than it looks. Maybe a steeper down angle then it looks. Maybe a little more broadside than it looks. Maybe a little slower than the shooter thinks.
Add some or all of those things together and you can wind up with a poorly expanded bullet clipping the bottom end of the lung(s). That makes the result a lot less surprising.
Bottom line: the shooter needed some insurance. ie more speed, less range, better hit, better angles. Lung hits alone with enough bullet speed to cause good cavitation will bust the lungs completely. The time it takes to run fifty yards is all that leaves them.
