Yup, that is typical flocking behavior. They will flock towards food, but when they push to the middle of the flock, it is usually to avoid a predator. As Andreanar said, this behavior is seen in guineas. They do it when they find a snake, so interesting! However, chickens flock like this because they sense, or see a predator. They push to the middle, the ones on the outside become the first victims. I only have a flock of 9, and I saw this behavior just this Summer, when a bobcat tried to enter their pen. I just happen to be cleaning up in the area. At first I thought they were fighting, but they weren't really making any noise. Then I saw the bobcat on the other side of the pen. He was hiding in some tall grass. Two weeks later, he took one of my hens as they were out of the pen in the back yard. Take another look, the hens know something is wrong. 
