With my son it was plain boredom. He's a fast learner and had to sit and wait until everyone else 'got it' before they moved on. Teachers started giving him advanced work to do while he waited. Perfect solution!
Teachers can be quick to diagnose 'problems' for any child who doesn't just sit there. One year, school got a new, young teacher. She strongly suggested ADDH testing for damned near every boy in her classes. Town doc decided to test her; gave some of the kids placebos, with parents permission. Teacher gushed about their improved behavior.
The boys didn't change, only her perception of them did.