Maybe he tripped and fell.
In that case, he may have been the one whose blood was spilled clearing out the prison.
Also, I knew something wasn't right in the earlier videos and photos because when they go in "hand-to-hand" he is up against the fence. I remember thinking, "I hope T-Dog doesn't fall against the fence and get bit."
I must admit that I didn't think that where he was in the circle was the most effective for killing zombies either. He would have to go through the rest of the group to get to the zombies inside the yard, unless they moved away from fence. If they were going to hug the fence, it would have been more effective for them to form a semi-circle than a complete circle. Of course, that is if you are trying to kill zombies. If you are trying to stay alive while your friends kill zombies, it is a shrewd move. The only safer place appears to be with Lori and Carol.