Based on my experiences with very severe hyperacusis, which is now much better, I have a theory.
So, I personally discovered my PEM was mainly caused by my tissues being physically incapable of resting, even when fully relaxed or sleeping. This is why I didn't get that sense of relief and rest that healthy people get when they rest. had no idea until cranial osteopathy treatments allowed my body to rest, and I was *shocked* at how PEM literally melted away the moment I began resting. You can read my blog post about it
here.
But my noise sensitivity got much worse.
It was already so bad that my mom (caregiver) had developed severe anxiety of being around me at all, as the slightest "ding" of a dish would literally have me screaming in agony. And then it got worse from that.
What I discovered for myself is that my noise sensitivity is worst when I need to rest. Just like a healthy person would get cranky at being woken up often by a noise they might not even notice during daytime hours. I realized I was being "woken up" from a state of rest my brain desperately needed. Even if I wasn't asleep. Even if I was sitting up doing stuff on my computer. Because my body was so flat-out exhausted from not being able to actually rest a single second for years on end that it was pretty much always resting as much as it possibly could, and noise jolted it out of that rest. The so incredibly desperately needed rest.
So when my body learned how to rest even deeper, the noise sensitivity got worse. Temporarily. After a few months of resting it began to get better.
These days, the initial sound usually doesn't cause my any reaction, but I still have a limit to how much repeated/continuous noise I can handle. I also have PTSD, and some specific sounds, like dishes clanking, now trigger flashbacks so that's fun too.
When I'm in a crash, the sound sensitivity can, for that day, spiral back to nearly as bad as it used to be. My body becomes just as desperate for rest, or the closest thing it can get to it, and the noise forces it otherwise. This is also why it can become a horrendous spiral - more noise leads to greater exhaustion, which means I even more desperately need rest, which leads to increased sensitivity.
All I know is this is the way it seems to work for me.