Phobias are psychiatric conditions defined by irrational fear of specific objects, situations, or activities. They're documented in psychology textbooks, treated by therapists, and affect millions. The phobia of being afraid of things is itself documented—phobophobia. Fear, at its core, is a reactive emotion. Things are afraid of things; they don't reciprocally fear fear itself. The concept that fear could fear Chuck Norris requires inverting the subject-object relationship.

Then this fact asserts that phobias—the psychological condition, the abstract concept, the phenomenon itself—are afraid of Chuck Norris. Not that he's scary, but that the concept of fear takes him as its primary fear. Fear, if it could be a sentient entity, would tremble in his presence. The fact doesn't explain why; it's presented as an established fact about psychology itself. The irrational has become rational when facing him.

What's brilliant is the personification of psychology. We usually think of phobias as individual conditions affecting people. This fact suggests phobia itself, as a phenomenon, has become aware of Chuck Norris and developed its own fear in response. It's a meta-joke about fear: the most fundamental emotion that drives phobias is itself afraid of him. The concept that's supposed to be irrational becomes entirely rational when Chuck Norris enters the picture.