Maybe it was flapping in the wind? Or he just was surprised by the shape of it? I don't know....Weird. Probably the same reason some of them are afraid of vacuum cleaners and blow dryers...Well those make noise and last time I checked aprons didnt'....hmmm I dont' know.

Maybe he doesn't like your cooking....? Just kidding!!!
Duncan is scared of those blue drop-box mailboxes Who knows?