An unread message becomes "he's angry with me", then "the relationship is over". A headache becomes a tumour. A mistake at work becomes dismissal, then not being able to pay rent.
The chain has two steps, and both are needed: overestimating the probability (how often does this actually happen?) and overestimating the consequence (if it did happen, would it truly be unbearable?). Anxiety inflates both at once, which is what makes the scenario feel inevitable.
It is a distortion, not a prediction. The practical test: write down what you fear will happen, then check back in a month. The real accuracy rate is the information that shifts the belief — not someone telling you that you're exaggerating.