fertility rate is falling in India for every section but at the same time one specific group still up in fertility rate and if we continue to follow this trajectory then big demography change is coming in next 20-30 years. Without sounding alarmist, one should remember what happens when some group reach 30-35% of total population or become majority.
I looked up the reports of fertility rate decline categorized by religion and all are declining compared to past, so I am not sure you are implying the religion or caste here. 
But this is a bit stretch and even if it's real it can't be planned and it must be due to the natural diet practices followed by them and vegans tends to lose it more than the others while we still have government propagating religious views and it could influence some to change from what they are supposed to do.