Did I miss something? What claims?

There are at least 4 reasons people experience a return of concussion symptoms.

1. Another impact causing another concussion

2. An impact causing a whiplash trauma

3. A PTSD or anxiety driven flashback that causes the mind to replay concussions symptoms, both physiological and emotional/psychological
Many in this group have a history of anxiety or depression or OCD or perfectionism or need to be in control.

4. An event that is over-stimulating, loud noises, too many voices, visual over-stimulation, a job task that is fatiguing with these excessive stimulations, and any of a myriad of these stimulations.

Some may have other underlying conditions with hormones or blood sugar or high blood pressure, etc.

Some may have done some innocuous activity that they would never expect to be a trigger. A late night or a bad night's sleep. Having a few drinks and getting a buzz. Even a lot of exciting and happy activities can be taxing on a compromised brain.

Try to pay attention to what your body, mind and brain is telling you. Learning to moderate your daily life to limit triggers is important.