Everyone thought that Hermione Granger's mindscape must be a place of detail and order. Considering her recall of facts in class and meticulous assignments, how could her mind be anything but organized? Snape was disappointed to learn that everyone was wrong. Granger's mind was a whirlwind of flittering thoughts, chaotic, and continually jumping.
As a headache bloomed behind his eyes, Snape cursed Dumbledore, the Order, and the Granger chit herself. He thought teaching her occlumency would be an easy assignment. Considering her annoying tag-a-longs, Granger was the sensible choice when it was mentioned that the golden trio needed to learn to protect their minds and Severus got first choice from the lot. He chose wrong.
How could anyone function day to day in such a busy mental state, he did not know. It did at least explain Granger's incessant fidgeting. With a mind that frantic, it made sense that she would need to expel energy physically. Snape recalled reading something about ADHD in some medical journal in the waiting room of the ER when his mother was still alive. Granger certainly fit the description of "continually running engine". Maybe she should look into that. Or maybe she already has. Either way, he was going to swap places with Moody or Shaklebot after this session ended. Either of the dunderheads that makes up the other two-thirds of the trio would be easier to teach occlumency. Snape wasn't sure where he would even start in the whirlwind of Granger's thoughts to start building defenses. He just had to make it through the next 30 minutes without his head exploding.