Some Black Angus cattle carry a red gene, but since red is recessive, it's generally not ever expressed in physical color of their calves when breeding to another Black Angus animal.  The reason for this is because the majority of Black Angus cattle have two black genes and very few are red carriers.  The only way for red color to show up is to pair up two red genes together in the calf.  Once again, since black is dominant over red.  Hence, the Red Angus breed was created.

So, if you have a red cow bred to a Black Angus bull and get a red calf from the mating, you've got yourself a Black Angus bull that carries both a black and a red gene for color.  Just happened that he sent along his red gene to the calf in this case that you're describing and that was matched up with a red gene from the cow.  Two reds equals a red :!:

Randiliana already stated most of what I just outlined.  Just couldn't help myself.    :roll: