His career track took an unexpected turn when he moved from business to non-profit work.

track 在这里用作名词，本义是"轨道、跑道"，引申为"路线、轨迹"或"发展方向"。 take a turn 是固定搭配，表示"发生转变/转折"，这里的 turn 指"转折点"或"变化"。 career track 指"职业发展轨迹/方向"，描述一个人职业生涯的总体路径和发展趋势。