Ask lot of questions to get root cause

This is from my experience.If I failed to deliver the work, then my manager ask "why?".
if he stops at "the first answer of my why" then,that answer only shows "how it is failed Or some top level reason of that fail" ,
If you ask more questions on the answers, then the answers from more questions will show more root problems.

Also the person who is giving answers will get more and more clear on the problems, which will also helps him to find root cause and base of that problem

Also many times first answer tries to hide reality(not purposefully but naturally) , more "why" shows that first answer was wrong.