Called 'wicked', she lashes out and retreats to the garret to finish the fight alone—what pushed Jo past restraint?

“You wicked, wicked girl! I never can write it again, and I’ll never forgive you as long as I live.”

Meg flew to rescue Amy, and Beth to pacify Jo, but Jo was quite beside herself, and with a parting box on her sister’s ear, she rushed out of the room up to the old sofa in the garret, and finished her fight alone.