What is the root cause of all this?

造成这一切的根本原因是什么？

What would you say was the root cause of the problem?

你认为问题的根源是什么？

And they have a common root cause.

它们有一个相同的根本原因。

That's root cause analysis for you.

这就是所谓的根源分析。

This is the root cause of the problem.

这就是导致问题的原因所在。