He doesn't like to talk about the time he won an entire bag of banana-flavored Laffy Taffy in a raffle.
On a more serious note: How long before Calamity are we talking? It was established that Megan and Sourcefield had their fears long before Calamity rose, but David didn't realize his fear of water until he arrived in Babilar—thirteen years after Calamity. Now, it was also established that Prof developed his powers extremely early—well before most other Epics—but if his fear of uncertainty, say, was exacerbated by the mysterious red star that appeared shortly before he got his powers, it could be a clue. Not that his weakness is Calamity himself, but it could be a hint as to his real fear.
I know I'm grasping at straws here, but still.