I think childhood fears are what establish a fear of clowns. Circuses have an unnatural, erratic strangeness to them, in a way that can often appear overwhelming to a small child, what with their lavish lights, odd music, and over-the-top performances. because this can appear so strange to a child, they might form a fear of it all, and because clowns are so vivid and memorable, they might be what first leaps to mind when someone recollects these fears. hence, the fear of clowns.

A thought.