This has happened to us more times than I can remember. Someone makes a seemingly innocuous statement or observation, DS hears it, and a few hours later his anxiety is off the charts.

Rational reasoning sometimes helps. But with anxiety, it won't always. For some things, exposure to a mild version of the fear can help. For instance, if your kid is afraid of riding his bike, try to get him to ride it for just one minute. After that proves that he won't die, maybe two minutes, etc.