Can estrogen dominance always cause uterine cancer?

No, a risk factor. Unopposed estrogen is a risk factor for uterine cancer but it does not always cause it. Unopposed estrogen means it is given without any Progesterone another female hormone. Somewhere between 4-8 out of every 1000 women receiving unopposed estrogen per year will get uterine cancer. Being obese probably makes the risk closer to 8/1000, whereas in thin women the risk is about 4/1000.