How come... ? 怎么回事？

How come 表示对方所处的状况“到底是怎么回事”（How did it happen?)，即“是怎么变成这样的？”，表示询问“理由”。可以像“How come you are late？” 一样加入更具体的内容。

例句：

How come? Do you have a schedule conflict?

怎么回事？你的安排冲突了吗？

How come she didn't show up last night?

她昨天晚上怎么会没来？

How come you didn't tell me about that?

你为什么没告诉我？

对话：

A: How come you're late?

你怎么会迟到呢？

B: I got caught in traffic.

交通堵塞呗。