Once the show revealed that it was Andrew who did all that. It pretty much proved to me why the guy was locked up in the first place.
But it is kind of funny how he was locked up for 8 months in the cafeteria during the zombie apocalypse and knew how to handle the walkers quickly. He even had an ax at the beginning of the ep. Where did he get that?
I also had doubts about Andrew (alone) settling up this trap but I took a closer look on the scenes and there are quite a lot of hints that could help proving it.
First of all to make things clear about the group not noticing the person who set all up: The first scene was certainly playing very early in the morning. I believe that the group was still asleep at that time. On top of this they sleep next to a bunch of zombies and have grown accustomed to noises like rustling fences or sth. walking around.
But now lets talk about the persons characteristics.
Like it was said before the trap-setter was a skinny black guy. From the characters we had known so far this could only be Andrew.
Also we get a good view on his shoes. So I tried to find a scene where his shoes were shown before but I only have seen it when they kicked down (episode 2) their first zombie and while Andrew fought with Rick (episode 4).
In the first scene the shoes were still way too clean but similar to the person in the beginning of episode 4 the person weared some kind of sneakers. So could have been the same pair of shoes.
But what IMO will settle most of the doubts is the scene where Andrew and Rick were wrestling with the axe. You can cleary see that his right hand was full of blood. Its impossible that his hands got this bloody from their fight up to now. We also saw the person lay down the deers heart before while his right hand was fully covered with blood.
So far I dont have left any doubts that it has been Andrew setting up the trap.
As mentioned by other users he still was a convict and in episode 2 he also learned a lot about fighting the undead probably.
Even if I am positive about this theory, I still have a feeling that maybe Oscar could have sth. to do with it.