I believe Crooks taunts Lennie simply because Crooks is jealous of Lennie. I agree with the response below that Crooks taunts Lennie out of anger, but I think the anger has a deeper root. Crooks sees Lennie as someone lesser than himself, yes, but there is more. Crooks wonders why this lesser human being is able to have lasting friendship with George. What does George see in Lennie that allows George to stay with Lennie? Crooks can't figure it out because friendship is not something Crooks has ever experienced before. Thus, the taunts about George not returning indicate Crooks' jealousy of the friendship between Lennie and George.
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.
I think that Crooks is very jealous of the fact that even though Lennie has a mental disability, he has a companion and Crooks doesn't. I think that Crooks wants Lennie to feel lonely and scared just like he does all the time. Crooks also wants to play with Lennie's head and see how far he can push him.