‘Reason’ [and its synonyms] is a term that, to use the words of W. B. Gallie, is ‘essentially contested”. It has a whole range of meanings in different contexts. The one you cite [case, explanation or justification of event] is a case in point. It usually is a response to a question such as “Why did he come to the meeting?” where the answer is held to embody the reason [e.g.”To make his case for a larger budget”]. In this sense of the term, reason is usually opposed to the notion of cause [i.e. efficient cause].
Reason is also used for the whole intellectual capacity of man or the specific intellectual capacity which allows us to move from one set of data to another. In this latter sense, reason is close to logic, being, as it were, the psychic source or ground of it.