I don't think it was a quarter crack that brought about Sportswriter's retirement. It was a combination of two factors.
(1)An inflammation of a tendon.
(2)The fact that the deadline for shipping a stallion down under for the forthcoming breeding season was imminent. If they had waited for the tendon to heal, they would have gone beyond the point where a down under stud career for this season was possible. The people with the down under money were there with the money in hand. Thus the decision to retire him was reached.