This depends school by school. At Wharton they typically want a detailed reason. A friend of mine deferred because her grandfather had cancer and she wanted to take care of him. Others I know have deferred because their ventures suddenly took off, and they wanted to continue working on that longer before starting school.
Bearing in mind that practically all schools have you sign honor codes before you enter (that also allow them to revoke your degree at any time, even after you've graduated, if they ever find you've misrepresented any information), I would err on the side of telling the truth. We're not un-understanding, but we want to ensure that deferrals are granted only in exceptional cases because you're taking slots away from other talented applicants next year.