Why did the astrologer share the old incident with his wife after many years?
The astrologer and his wife appear to have a close relationship. No doubt the astrologer shares almost everything with her. They lead a very simple life. He brings home money every night and undoubtedly tells her about the events of his day. But he has kept one great secret from her for years. He has never told her about how he had to flee from his village because he had stabbed a man and left him for dead. Obviously he does not want his wife to think that he is a murderer and that he might even be in danger of being arrested and imprisoned or executed. But when he finds out that the man he stabbed is still alive he feels free to confide in his wife at last.
After dinner, sitting on the pyol, he told her: "Do you know a great load is gone from me today? I thought I had the blood of a man on my hands all these years. That was the reason why I ran away from home, settled here, and married you. He is alive."
Of course, the astrologer is also relieved to have gotten rid of the avenger who had been searching for him all these years in order to kill him. By convincing Guru Nayak that he truly has mystical knowledge, the astrologer is able make him believe that the man he is seeking is dead, and he is also able to persuade him to stay in his village and never travel southward again.
It is a thoroughly happy ending. Guru Nayak gave the astrologer a whole handful of coins. The irony of the situation is that the astrologer really knows nothing about his profession.
...he knew no more of what was going to happen to others than he knew what was going to happen to himself next minute.
However, it just happened that he was able to appear to have considerable knowledge about Guru Nayak. The astrologer's real, factual knowledge of an incident that occurred years ago and hundreds of miles away made him seem to be what he always pretended to be--a man possessed of real supernatural wisdom and extrasensory powers.