"I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know. Unfortunately, that day has come."

This makes it sound like it does have to do with his health unfortunately, and not that he just feels that it's the right time to do it. (like Bill Gates did with Microsoft)

On another note, even though in reality it's not just for the Board but, as put in the letter, for "the Apple community", it feels odd to imagine that he would need to write a formal letter of resignation.