The story opens with a description of the astrologer's so-called "professional equipment." It consists of

...a dozen cowrie shells, a square piece of cloth with obscure mystic charts on it, a notebook, and a bundle of palmya writing.

This all sounds like second-hand acquisitions. The author does not tell why the protagonist decided to become an astrologer, but it seems likely that he had an opportunity to obtain these items from another man and that he considered them a great asset because he was poor, ignorant, a newcomer to the city, and didn't have any other way of supporting himself. He probably became an astrologer before he got married and had a child. He could hardly have gotten married if he didn't have a way of at least supporting himself. 

One can imagine that the original owner of the professional equipment gave up because of fierce competition. He might have even died of starvation, leaving his pathetic possessions to be picked up by the first passer-by, who happened to be the protagonist. The bundle of palmya writing may or may not be authentic, but it sounds as if it would be old and may have passed through many hands.

The protagonist may have tried attracting customers as an experiment and found that it was easy for him to play his adopted role.

He was as much a stranger to the stars as were his innocent customers. Yet he said things which pleased and astonished everyone: that was more a matter of study, practice, and shrewd guesswork.

Many of us are not too different from this astrologer. We get out of school. We need money. We take the first job we can get. We become something we never really intended to be, but then it is hard to change. And it becomes especially hard if we have dependents. 

In Narayan's short story "An Astrologer's Day" we find that the main character, the nameless astrologer, had never intended to become an astrologer. He admits to knowing about the stars as much as the man next door. He also never thought he would ever do anything of the kind as a way to make money.

Yet, money has to be made. He has a wife, and needs financial stability just like everyone else. Hence, he realized once that he has the ability to speak to people, to sense their worries and to help them feel better with his words. Apparently his unique touch soothes the fears of people, and he basically is a good educated guesser.

Therefore, two factors motivate the main character to become an astrologer: The fact that, as any other man, he needs to find a way to make money and support his family, and the second fact that he does have the gift of the word. What he says does a favor to others, even if it is not accurate, nor if he is really predicting the future. We find this towards the middle of the story where it reads:

...he had not in the least intended to be an
astrologer when he began life ; and he knew no more
of what was going to happen to others than he knew
what was going to happen to himself next minute. He
was as much a stranger to the stars as were his innocent  customers. Yet he said things which pleased and astonished everyone : that was more a matter of study, practice, and shrewd guesswork. All the same, it was as much an honest man's labour as any other, and he deserved the wages he carried home at the end of a day.