From what I understand, stress intolerance is due to a failure of part of the brain that filters out excessive stimuli so that the remaining stimuli is within the brain's ability to process. A large amount of brain processing energy is spent filtering out this stimuli. When this failure happens, the rest of the brain is overloaded with excessive stimuli. This is made even worse with PCS because much of the brain is already struggling to function.

There are likely other issues at play but this is likely the most problematic issue.