Probably doesn't understand why all these people and camera are making a fuss. I got the same deer-in-the-headlights look when I was a flower girl at my uncle's wedding. I knew my job, I had done the rehearsal and then when the moment came.... all these people were looking at me. It didn't matter that they we smiling or that I was related to most of them. Suddenly the church was huge and there was a sea of people looking at me, and I was terrified.
(Thankfully, I didn't cry, but I kept my head down the whole walk up. The camera man and my grandmother kept trying to get me to look up and smile, but nothing was taking my eyes off the floor until I was safe at the altar.)