what caused george to make fun of lennie

Are you referring to a particular point in the book or just generally?

book

Generally, George would call Lennie things like "dumb bastard" . He doesn't have malice in it though. I recall one point where upon telling the ranch owner that he was related to Lennie, George later jokes he would kill himself if they were related. Again, there is no malice involved, his teasing is part of their relationship. Lennie seldom comprehends it anyway.