TV Tropes has an article on the subject [http://tvtropes.org/pmwiki/pmwiki.php/Main/Nosebleed], it seems to be derived from Japanese folklore (and apparent true in rare cases). Essentially, it's the Japanese idiom for a guy seeing something arousing. The western equivalent is probably a guy acting like a dog (panting, drooling, howling, etc,), but fundamentally it's the same concept.
