Root cause is not the first thing that failed. It is the underlying reason that failure was possible.
The Whys technique:
Why did the server crash? Memory exhausted.
Why was memory exhausted? Log files grew too large.
Why did logs grow too large? Log rotation was not configured.
Why was log rotation not configured? Not in the standard build.
Why is it not in the standard build? Never added to checklist.
Root cause: Missing log rotation in server build checklist.
Fix the root cause, not just the symptom. If you only fix the symptom, the problem returns.