I've also had that happen on a buck about 5 years ago. My theory is that the speed and force of the bullet hit the esophagus and pulled semi digested food up out of the stomach. My shot was exactly like yours, broadside, right behind the shoulder. Shot entrance and exit not even close to hitting guts (and they weren't hit), and talk about stink!!!
You did the only thing you could do by cutting away all affected meat.