I feel there could be many causes for such a state of affairs in India where most secularists are born from Hinduism and end up criticizing Hindus.
- Avenue for self criticism being available only in Hinduism: Only in Hinduism we have multiple ways and paths of worshiping different God forms, different schools of faith, philosophies etc. Contradictions between one school of faith to another always existed. People in Hinduism enjoy lot of freedom in engaging in arguments, criticisms and denials of practices and philosophies. Such a freedom does not exist in other religions.
- Born and brought up in elite class: Where parents are English educated, belong to socially upward class, having admiration for western culture, erstwhile beneficiaries of British Raj, having no deep-rooted faith or respect in Hinduism or its spiritual aspects, then their children may end up as pseudo-secularists. Having born and brought up like that, they do not get any exposure to Indian Hindu cultural traditions, values or scriptures. Money, position and status become the yardsticks for them for success. They don’t come across any situation needing a deeper understanding or appreciation about true Hinduism. They look at the superficial weaknesses of the religious practices and pseudo-sanyasins and start developing secularism as a better idea.
- Suffering under the class and caste domination of Hinduism: By experiencing the evil effects of untouchability, upper-caste vs lower caste discrimination in life, not getting any chance to come across proper spiritually evolved Hindu saints or sage in life, by getting cheated by dubious Godmen and so on, some people develop hatred on Hinduism and they start believing secularism to be of a better value.
- Negative effect of too orthodox and strict parenting: If by chance the Hindu parents are too orthodox, too strict and critical about their children, do not offer any logical replies to children asking questions about their faiths and practices but only demand obedience, then their may children grow up antagonistic towards parents. Their hatred extends to the parents’ religious beliefs and practices too. They develop a negative mindset about the religion itself. They won’t be inclined to study deeper into Hindu religion and spirituality. They find secularism as the right choice and criticize Hinduism openly based on their past bitter experiences.
To conclude, Hinduism does not feel threatened by criticism and denials. Hinduism is very broad minded. One can bitterly criticize Shiva or Vishnu or any other Avatars or saints and still live peacefully in India. It is not possible for a Muslim to criticize Allah or the Prophet or any archaic practices of Islam openly and hope to remain alive and safe for long.