A proximate cause is the event most closely associated with or immediately responsible for an observed result. This may differ from the root cause. Getting to a root cause often requires a deep dive into the situation using approaches like Socratic questioning or the 5 Whys
Post-mortem reports for critical failures generally outline proximate causes and how they stem from the root cause. For instance a website was down due servers running out of disk space which was in turn a result of a bug which in turn was a result of poor testing and so on.
A child repeatedly asking "why?" to answers for an initial question is trying to get to the root cause.