There are different theories as to what is PCS or what causes PCS.

My personal favorite is that PCS is a relatively minor Diffuse Axonal Injury or DAI. A severe DAI would cause death. But the idea behind it is that many axons were torn (actually that their myelin sheaths were damaged) and they are taking their sweet time fixing themselves or re-routing pathways that are irrepairable.

Another theory I've read is that PCS is (again) relatively minor damage caused by a concussion to some part of the endocrine system - like the pituitary gland, thalamus, hypothalamus, or pineal gland because symtoms caused by issues with one of those areas of the brain are very similar, if not identical, to PCS.

And yet a third theory I've read is that it is an indication of multiple concussions, or even sub-concussions (which are jolts to the head that help the brain acquire some very minimal damage, but not enough to actually make the person aware that there is any damage) but because of the cumulative nature of brain injury, it all just manifests into PCS symptoms.

Experts really don't know what PCS is. There are a lot of theories, even more than I've listed, but I don't think I can recall any other ones right now. But they are trying to figure it out. Frankly, all of those theories could cause PCS or none of them could.

I've been doing a lot of layman research about all of it.

Many symptoms could be caused by fatigue, which might be caused by lack of activity or the fact that the brain has to use more energy to heal. As we all well know, many of the symptoms could be caused by something psychological - a chemical reaction in the brain that alters its functioning. It could very well be that a very minor increase in intracranial pressure from swelling caused by the initial injury could cause a lot of symptoms but that most people's brains or bodily systems are able to regulate it and absorb the CSF.

Our society really needs better technology to observe smaller pieces of the brain while the person is still alive and the brain and skull are still intact. I'm very hopeful of this diffuse tensor imaging research that's being conducted to try to identify what's going on inside of people's heads on a biological level when they are recovering from a concussion.