No, I don’t think IIM Kozhikode's selection criteria are biased in favor of students from Bengal. One of my uncles was on the interview panel a few years ago. He shared that shortlisting good candidates in Bangalore was a difficult task. Very few candidates were suitable enough to get selected. On voicing his concern for the same he was told not to worry as they will surely get more than enough candidates from Kolkata.
The conclusion is that whatever the reason, there is no special bias towards students from East India. It’s just that a lot of them clear CAT.