It seemed obvious to him. "Well, first it was because people were trying to find a safe place anywhere. There are a couple of roads around Atlanta that were just full of cars of people trying to get away. After that it's because the walkers got inside. Once they know you are there, they don't stop. They broke fences around the prison because they would just crowd in and never step back, even after hours of trying to reach through it. You have to be really quiet to convince them you aren't there."