I wouldn't be too concerned with the why, it happens. The bigger concern is getting rid of it - antibiotics don't get into the prostate well and it can take a long course of heavy antibiotics to get rid of an infection and/or a man can end up with a chronic low-level infection. On the other hand, it could be a course of antibiotics and it goes away. (I'm not any kind of expert, this is what I learned from my ex- and his doctor when he had a prostate infection.)
Julie