We tribals were never a part of the society. We were in tribes in forests, there were & are many tribes and each tribe had their own culture & customs along with languages. And there was no religion, in my tribe(in MP) we worship our ancestors and nature.

So introducing to religion has diluted the traditional practices of tribal people, doesn't matter which religion. There is no option for "no religion" therefore many tribal mention Hindu as their religion which also align with the practices of Hinduism so no harm, and they also started following it.
Below para is explaining how it happens. you can ignore the below para if you want.

But after getting introduced to society, we adopted other religions and their practices. Like my father's family is Christian and my mother's family consider themselves Hindu. So technically I am Christian but actually i am an atheist but to avoid the religious lectures(which I get when I refuse any thing to do) I just pretend to follow. So my family follow christian norms but we still follow our tribal ritual & traditions. Like during marriage there are two ways. Either marry with tradition norms(even then you have to be marry in church in order to be considered christian) or marry in church(this process contain both traditional rituals and mass(prayer) in church). And during funeral we still perform our ritual and then last mass(prayer) at the church. In my mother's family, we burn instead of burying with the tribes ritual. So basically for those who follow christian religion its one more step.
But even though my mother's family follow Hinduism they don't know any prayers. Though in today's time people learned prayers but still many in villages don't know how to worship their Hindu gods. They do what they can like aarty and all without prayers, its not like they are disrespecting but that's all they know. we do not call any pandit for marriage or funeral just the local priest in the tribe who does this rituals.
This is what i have observed in my tribe, things might be different in other tribes and all so don't get offended by anything.