Don't get the wrong idea -- I too am very skeptical! I have always dismissed the experience as an active imagination. But you can imagine my surprise when I read that what I experienced is a widespread phenomenum, down to the hat and trenchcoat and behavior. Happenstance? Cultural influences? Something hardwired?
Maybe I took my explanation in a wrong direction: I was not intending to dismiss your question as if you were believing in some supernatural stuff (nor, for that matter, assuming that you were abusing substances as a child :P). I have never experienced something like it and got very inquisitive upbringing, hence maybe I was not really prone for it as a kid? I don't know, but I do find question as intriguing. To your comment, I would rather lean away from 'hardwired' word. I don't know what kind of predator (aside of pedophile Columbo) could possibly be ingrained in humans on the basest of base levels of threat detection to be considered as hardwired. There is a phenomenon called pareidolia that was observed as a form of hyperactive/idle visual recognition side-effect in humans where we see people, faces etc where there are none. Since children are on a very receptive-intense learning phase it could perhaps run haywire. It's a person with unknown features that is not supposed to be there. It's foreign, therefore could pose a threat. That leads to fear, if you don't mind me going on a Yoda's monologue ;). I would like to get some better explanation though, this is just a fast-talking attempt at finding a good reason for Shadow People.