The term 'underlying reason' can be broken down into three crossword solutions: RATIONALE (9 letters), CAUSE (5 letters), and ROOTCAUSE (9 letters). RATIONALE refers to a set of reasons or facts offered in support of an idea, opinion, or belief. CAUSE is the source or origin of something, while ROOTCAUSE is the fundamental CAUSE of a problem or event. All three terms refer to the underlying reason for something, which is the primary factor that leads to a particular outcome.