My child is an atheist. How did this happen when I raised her as a Hindu Brahmin?

Your child went to school, socialised and got in touch with other aspects of the world etc -- and made a different choice in terms of faith and beliefs. It happens. Parents can lay all the groundwork and foundation but if the child comes to his own conclusion and decisions in the future, it's inevitable. You're not in the wrong, he is not in the wrong. It is how it is.