Boredom, I guess kids get that way sometime, although I really don't remember that happening to me, most of the time I was busy as a kid, and did not want to stop what I was doing, that is when I would get in trouble.
It seems as though your chat has worked for him, I bet he really enjoys the karate lessons.
Yes, my son is like that too. If he's occupied, he will be very well behaved. The moment he's not being entertained, however, is when he gets more mischievous.