I looked around last night after reading your post, and I found a couple of pictures. It seems like I've seen several pictures of fire damaged box turtles before where the shell "bubbles up". It looks like the scutes almost melt. In the pictures I found last night, though, it looks like the turtle that you found.
I think fire damage is entirely possible. I just think that the margins of the damage are too clean. I've seen some pretty wicked fungal infections of turtle shells, and some look a lot like the turtle you found. There are also scattered reports of people setting fire to turtles on purpose...I guess that's always a possibility. Does the area where the turtle was found have a history of fires? Where I live now, we typically don't have forest fires. Where I grew up, they were a yearly occurence. Amazingly, I never found any turtles that appeared to be fire damaged out of the hundreds that I saw.
Here's one picture, and two links to other pictures. I can't get all three to load on here. If you can access some old Herp. review texts, Dodd apparently has done some thorough studies of fire damage in box turtles.
http://www.biology.duke.edu/dnhs/pics/Boxturtle2.jpg
http://www.snakesandfrogs.com/scra/articles/images/burnedbox.jpg
Burned Ambo.
