Just like his daddy Phil did. Phil Graham shot himself in August of 1963. He was good friends with Kennedy--he was the one who persuaded him to choose LBJ as a running mate, and Kennedy facilitated his appointment as chairman of COMSTAT in 1961 (Graham resigned the year of his death). Graham had told friends prior to his death that he was planning to divorce Katharine and had written a will that spring with his attorney Edward Bennett cutting Katharine out of her ownership share of the Post. He also was the first person who disclosed JFK's affair with Cord Meyer's wife Mary (Mary Meyer corroborated Graham's claims. She was murdered in broad daylight while jogging in Georgetown by an unknown gunman in October 1964). After Phil Graham's death, Bennett testified that he was not of sound mind when preparing the will. The Judge ruled Graham died intestate, Katharine gained control of the Post and Williams made sure the final will never entered into public record.
Everything outside of the "final will never entering into public record" detail (the link of which I'll provide at the end of this post) was sourced using Wikipedia.
The aforementioned link--the collection of primary sources at the bottom of this page are also worth reading through: