What reason does Curley's wife give for going to the bunkhouse?
Curley's wife is a "tart" - a slang term for a woman of dubious, possibly promiscuous behavior. In short, she's trouble, and George knows it, hence his attempting to stay away from her and counseling Lennie to do the same.
She supposedly goes to the bunkhouse to look for Curley, but this is a lie, or at least a thin excuse; she knows perfectly well where Curley is. She's really just looking for company and conversation, or rather, someone to listen to her complaints. The way she mistreats and verbally abuses the men evidences her dissatisfaction and self-loathing, and makes the men reject her presence even more vigorously. Curley's wife is very much like a child who wants any attention it can get, even if that attention is negative.