Doesn’t directly answers your question, of which I suspect parenting very high on the likeliest reason — as that is the reason I’ve seen others provide for him stepped down and out from YC. But you can see from his last major period of activity https://news.ycombinator.com/threads?id=pg ends with his departure blog post: https://news.ycombinator.com/item?id=7493856 and the top comment is his wife hinting that Mr Graham was overly consumed by managing hacker news ;)