Subject + 是 + 怎么回事
怎么回事 (zěnme huí shì) means "what's going on" or "what happened," used to ask about the cause or nature of a situation. It is a common conversational expression for seeking an explanation. For example, 那个是怎么回事 means "what's that all about?"
怎么回事?路上怎么这么堵?
Zěnme huí shì? Lù shàng zěnme zhème dǔ?
What's going on? Why is the traffic so bad?
你脸色这么差,怎么回事?
Nǐ liǎnsè zhème chà, zěnme huí shì?
You look terrible. What's going on?
我不知道是怎么回事,电脑突然就死机了。
Wǒ bù zhīdào shì zěnme huí shì, diànnǎo tūrán jiù sǐjī le.
I don't know what happened — the computer suddenly froze.