Hermione had always known she was a bit of a know-it-all.
So when she came across something she couldn't do, it made her quite irrational. Sport had always been her Achilles Heel, even in Muggle school. So when the First Years were required to learn how to fly broomsticks, she very nearly flipped out. Flying! Instantly, she knew she was going to be terrible at it, and her nervousness showed as the people around her became irritated.
She never told anyone how much she'd have given to fly away from them all.