is it for real? what is the source? I have heard of Kamakhya temple in Assam famous for animal sacrifices. But they slaughter Buffaloes and goats. Ritual animal sacrifice is not very uncommon in India (goats and chicken). But never heard about cows.
I m not an expert in religious subject but here in NE india most hindus (brahmin and aryanised natives) follow what is called as shakti-sect (power and might etc) of hinduism or something similar and the above ritual has something to do with some legend of a Rakshasa (Narakasur probably) being born out of buffalo blood..
But TBH i think the real reason is that the ritual is just an adoptation of earlier tribal/animist rituals practiced by natives and incorporation of such into hinduism....
Similar ritual are still practised in Tripura ,Arunachal and some part of upper assam I heard
edit: BTW it is always Buffalo never cow and it has stopped now