Please submit the reason for your absence in writing.
The meeting was postponed due to unavoidable circumstances.
Due to unavoidable circumstances, the meeting was cancelled.
The application form has a section to explain the reason for resignation.
This conduct falls under the grounds for dismissal.
原因 points to a direct, often causal, source of an event (like an accident's cause), whereas 事由 is a broader formal term for the grounds or circumstances behind an action.
事由 (grounds/reason) and 自由 (freedom) are pronounced identically as じゆう but have completely unrelated meanings, so context and kanji are essential to distinguish them.
justifiable cause for noncompliance with the law; grounds for justification
Compound of 事 (thing, matter) and 由 (reason, cause). The combination has been used in formal Japanese to denote the reason or grounds for something.