Hm, good question. I don't know. I think that to some degree if our phobias were rational, they wouldn't be phobias.

Or else something happened when you were young that you don't remember. That happened to my mom, with her fear of circuses and clowns. She never knew why til one day she mentioned it to her brother -- he remembered the bad day at the circus, but she'd blocked it out.

I've had, and still have to some degree, some weird fears too -- aliens/outer space, robots...

Or maybe it's a past life thing, if you believe in reincarnation. ;)