他突然改变主意的背后动机是什么？

What was the motivation ___behind his sudden change of heart?

他辞职背后的真正原因仍然是个谜。

The real reason ___behind his resignation remains a mystery.

贪婪是这次公司丑闻背后的主要驱动力。

Greed was the main force ___behind the corporate scandal.