How do Troy's unyielding expectations result in his downfall? I have identified his expectations as a father and a husband, and I need one more argument for him as a tragic hero. Please provide examples.

A tragic hero in literature is one who has flaws to which he or she is blind. These flaws bring about his or her fall. Troy in August Wilson's play Fences is a tragic hero. As you write in the question, he has unrealistic expectations about himself as a father and husband. He believes that if he works hard, he can protect his family from misfortune, including not only poverty and racism but also death itself. While trying to protect his family, he also alienates them; for example, he does not want his son Cory to play football because he fears that Cory will be the victim of racism. He does not want his older son, Lyons, to be a jazz musician because he fears Lyons will not succeed. In trying to protect them and steer them toward "safe" futures, such as working at the A&P or on a sanitation truck, Troy alienates them and drives a wedge between himself, his sons, and his wife, Rose. He also cheats on his wife and does not meet her expectations of being a good husband.
Like the tragic hero, Troy is blind to his faults. Perhaps his greatest fault, though, is that he believes he can defeat death by building a fence around his house. In this sense, he has unrealistic expectations of what it means to be a human being. By constructing this fence, he also makes his family alienated from him. In the end, death claims him, and he falls, like a classic tragic hero.