In my opinion, Crooks taunts Lennie because he (Crooks) is a man who is angry about the way that he has been treated by all the other people on the ranch. Because he is black, he has been treated badly by everyone and so he wants to get revenge on someone.
They say that everyone wants to be above someone in their life. If you are picked on by everyone, you want at least one person that you can pick on so you don't feel like a complete loser. I think this is why Crooks taunts Lennie. He senses that Lennie is in some way lower on the "totem pole" than he is and so he will hurt Lennie to help make himself feel better.