Think of the reaction as a small program sitting quietly, waiting for one particular condition. It is not running all the time, and which condition wakes it is a question with an answer.

What the research says

In a treatment study across mixed anxiety and depressive diagnoses, difficulty tolerating uncertainty was elevated at the start, fell during treatment, and its reduction was associated with better outcomes. Not knowing when the thing will fire is part of what makes it heavy, and that part does seem to move. This was a secondary analysis, so the link between the two changes is an association rather than a demonstrated cause.

Strength of evidence: one line of research. A single study, or a theoretical account.