The answer is because he doesn’t listen to his parents and that’s considered cursing them, the punishment for cursing the parents is sekila. So he is being punished for this sin. Baal tosfot.
That’s very nice, except for the simple fact that there are definite parameters to cursing one’s parents and simple disobedience or even direct disrespect, while being forbidden, does not equate with cursing. We don’t stone people who simply disobey their parents.
The Wolf