In any large gathering of people, the thing to fear most is chaos and commotion, because it easily sparks panic, and once panic spreads, it quickly leads to a stampede.
In a stampede, once a person falls and gets knocked down, getting back up in the middle of a crowd is practically impossible. If you fall in a moving current of people and don't know how to protect yourself, the consequences are terrifying.
At Hogwarts, because massive brawls have broken out so many times—you could almost call them a daily routine—anyone who isn't a fresh-faced first-year is extremely experienced.
Chapter 442