Ambedkar feared unconverted Adivasis could become an anti-Hindu bloc if non-Hindus converted them

Evidence

The Hindu has not realised that these aborigines are a source of potential danger. If these savages remain savages, they may not do any harm to the Hindus. But if they are reclaimed by non-Hindus and converted to their faiths, they will swell the ranks of the enemies of the Hindus. — Ambedkar, B R, Annihilation of Caste_ The Annotated…, 363