Like others said, it's not exactly known and hence why controversy shrouds this hell syndrome. A family GP gave me a recent clinical literature review on PCS which said cohort studies tend to consistently find female gender and increasing age are risk factors. They also talk about increasingly finding that sport related concussions have better natural recovery than those caused by MVA, falls and assaults (things more traumatic and unsuspecting). They also talk about the likelihood of PCS stemming from structural/biochemical changes to the brain from injury as well as psychogenic origins. There's also the debate of PCS persisting if the individual is more of an intellectually stimulated sort. (At least we can consider ourselves smart!

This is all speculative science of course.. but thought I'd share what I've found from a reputable source