What is the motivation behind this sudden change?

　　這種突然轉(zhuǎn)變背后的動機是什么？

　　The sudden noise made her start.

　　突如其來的聲音嚇了她一跳。

　　His heart gave a sudden leap when he saw her.

　　他看見她時，心猛地一跳。

　　The children were surprised by the sudden entry of their teacher.

　　老師突然進來使孩子們感到意外。