Just to add, my money would be on estrogen dominance also.
As has already been pointed out, the conventional wisom is that prostate problems (BPH and cancer) are caused by testosterone (particularly DHT). Yet prostate problems are more common in older men, who have lower free T and higher estrogen, which makes estrogen a more likely candidate.
It has also been pointed out that testosterone stimulates XMRV (did this come from the prostate cancer studies?). However, as I understand it, many receptors can be bound by either testosterone OR estrogen and in fact estrogen has a greater binding affinity. Meaning that, when both are present, estrogen is more likely to bind (and block) a given receptor.