Can an Atheist be angry at God? I doubt they would call it that, because to have anger at someone you have to believe they exist. Still, in some of my dealings with Atheists, it becomes clear to me that they do express an anger toward God. It doesn't look like the kind of anger that a believer has. Believers get (in my unscientific study) angry at God and huff and puff for a while, stop going to church for a bit, and then come back to the faith because they realize, in the end, that God does exist and that He's good. I myself have been extremely angry with God in the (recent) past, yet I never doubted His existence. I suspect that Atheists tend to express their anger at the church organization, individual Christians, parents, or what have you, but the fact is that God is the ultimate subject of their anger.
CNN, which has long been thought of as a liberal news source (and it tends to be), reported on the findings of Case Western Reserve University's study.
I know that several of my unbelieving friends would loathe to admit it, but I suspect that some of them might be angry at God, particularly through the church, Christians, ect. In fact, I know of one such man who expressed pain brought upon him by the church and how the history of Christianity is tainted with bloodshed. So this anger, which ultimately leads to rejection of God, is based on the surface against those who do believe in God.
Most Atheists would disagree with me, and I accept that. I have several friends who are atheists, or at least severely agnostic, and I know that they are what we would call "good" people. Still, for many of them, there is an anger in their hearts, as the Case Western Reserve study shows.