That's to bad. I know the feeling, it sucks.
I really doubt that this was an impaction issue from just feeding. Just a guess but I would bet the substrate in its mouth is from the moments before death. I lost a GTP that seemed to be having a seizure one morning when I was checking on my collection. It was on the cage floor twitching, spinning, and squirming. As it did this it got a ton of substrate in its mouth. I got it out and removed the substrate and it died in my hands.
As for the actual reason for the death I just don't know but Im interested to see what you find out.