[Hawks Coach]

Could be a bias, could be rookie syndrome, could be they thought the other ref was more "workable." You can never know.
I have seen rookies handled in two ways. One is to ride only the rookie hoping he'll crack and give you some calls. The other is to ride his partner hoping he'll take pity on you for the "mistakes" his inexperienced partner is making and give you some calls. I am not sure that these techniques actually work, but coaches are coaches.

That said, I see bias all the time and it is naive not to suggest that it exists. I work with a man who won't sit down at lunch with our coworkers when it is just the females, will readily sit down if another male is already present. I have worked with another man who would direct questions of a technical nature only to males, never to the females, even when the females were the technical experts in the area that he need info (and the women knew it!). I have seen similar dynamics between blacks and whites, and both directions. The bias may be a conscious prejudice or an unconscious bias that the person is not even aware exists. But if biases are evident in the workplace, they exist everywhere and will be there on the court.

I also see plenty of people who demonstrate no bias in this regard. I would like to include myself in this category, but I am not naive enough to think I am qualified to judge myself. We all have preconceptions we carry around with us and they come into play in the most subtle ways at times.