。
She quickly turned around to look behind her.
Literal
She [topic-は] quickly behind [object-を] turned-toward.
振り向く is a compound: 振る ('swing / wave') + 向く ('face / turn'). Together: 'swing one's face toward / look back over the shoulder'. The を on 後ろ ('behind') marks the direction of the turning motion — を with motion verbs covers paths, exits, and angles of movement, not just objects acted on. The full pattern 後ろを振り向く means 'turn around to face behind oneself'. すばやく adds the quickness.