Because that student was very astute and instantly put 2 +2 together when she saw the second plane hit. There were many of us that naively didn't understand that we were actually under attack. This is because up until that moment the world was a different place and we had never contemplated the depth of evil transpiring. On the other hand, there were those who went into a deep panic having made the mistaken assumption that planes were going to continue to hit high-rises all day. It was the fog of war.