It's none of their business and honestly, I doubt they care as to why you're leaving. Just make sure no bridges are burnt if you're gonna put this job on your resume and list references.
Thanks. I'm on good terms with my manager, so it shouldn't be a tough talk.
One last thing: she leads the management team, rather than being one sole manager. Do I refer to each member, or should I refer to her? If the former, do I list down their names individually, or does "Dear Management Team" cover it?
One last-last thing: I have never referred to them by their last names (pretty casual work environment), should I address them by their first names (Dear John vs. Dear Mr. Smith), or really stick to last names?