Through some kind of nervous system reflex, the deer bit off the last two inches of his tongue apparently at the moment he was shot. Due to the high impact velocity and the heavy bone/muscle, the bullet came apart and did not exit.
Roskoe, I make no illusions about knowing more than you about hunting/ballistics or anything else. You've helped alot of people here and me in particular. This is something that really doesn't matter and is just nit-picking. The tongue being bit could very possible been the sheer physics of the situation. Go ahead an jab yourself in the adam's apple

it'll drop your jaw and lift the tongue up-and-out...then if a bullet travelling several thousand FPS tugged/crushed you neck bones back and slightly downward, after jabbing you in the throat, your skull would come down on your jaw pretty hard...and your tongue would probably still be in the way. Just a thought