I fucking hate it when people are just jumping in their seats for no goddamned reason. Not listening to music or anything, just bouncing up and down while reading or writing or using a computer. I'd feel bad if he had some disease or something, but I'm pretty sure he's just a douchebag who can't stop moving his knees and head.
And he's one of those types that'll just stop, let their jaw drop slack, and bend their head to the side violently to crack their neck. All that sitting and looking forward is damn hard work.
At least he's not stretching his arms and legs all the way out in all directions, and making grunting noises and loud yawns while he's doing so. I knew a guy who did that in like every class, constantly. That dude must have had some kind of disorder, he was just fucking WEIRD.