why does scout start fighting jem?

Jem pulls Scout out of the room and tells her to try not to get on her aunt's nerves.

Scout bristles at Jem's assumption that he can tell her what to do and his insistence on talking like he's so much older and wiser than her.

When he threatens to spank her if she misbehaves again, she flies at him, taking advantage of his weak position to pummel him - her holding her own in the fight, she thinks, shows that they are still equals.

source