
to a certain extent the risk of Pyo is age related, as it is the number of seasons and the long periods of progesterone influence that make it more likely to occur, so the more seasons the more risk.
Progesterone levels stay high after a season in unmated bitches for even longer than pregnant ones.
http://www.thedogplace.org/Reproduction/Estrus-Bitch-Wolf-Barber-DVM.aspPyometra & Uterine Infections
How does this happen? The bitch's ability to ward off infection in her uterus is highest during estrus when estrogen levels are high. This makes sense because this is the time when her reproductive tract is being exposed to a multitude of pathogens (and for any that doubt, just take a look at the end of your dog's penis). It's a miracle that the uterus can eliminate billions of bacteria without harming the delicate sperm cells.
In contrast, uterine defense mechanisms are minimal while under the influence of progesterone when the uterus "thinks" it is pregnant. It is helpful (albeit an over-simplification) to think that when a bitch conceives and becomes pregnant, the uterine immune defense systems must be turned "off" so that the antigenically foreign conceptuses are not destroyed. If this did not happen, then the embryos would be rejected and destroyed in much the same manner as rejection occurs in mismatched organ transplants. When the uterus "thinks" it is pregnant, it fiercely protects whatever lies therein. And that whatever could be puppies in the case of a normal pregnancy, or it could also be pus in the case of a pyometra.