In my experiences, a LoA (when not directly requested by someone for personal or professional reasons) results from:
"We're choosing to separate our employment relationship."
"You're firing me?"
"If you choose to look at it so abruptly, yes."
"I've done nothing wrong. I'm going to sue you."
"If that's your choice, fine, but if it is we're putting you on leave until the matter is resolved. Think it over."
And it gets ugly from there.
Do I know for sure what happened here?
I'm not Tom Buning's physician, nor am I his boss.