。
She turned around and looked back.
Literal
She [topic-は] behind [object-を] turned-and-looked.
振り向く ('to turn around, to look back') is a tight compound verb: 振る ('swing, wave') + 向く ('face') — to swing oneself around to face. 後ろを振り向く ('turn around to face behind') doubles down with 後ろ ('behind') as the direction; the を marks 後ろ as the direction-faced, the same を-with-direction-verb construction used in 向こうを向く ('face the other way'). The whole gesture is small but loaded — a quick over-the-shoulder glance, often used in narrative for moments of recognition, surprise, or hesitation.