I've got to imagine that the nonchalance and disrespect of the interaction had something to do with it. I'm sure if you'd been next to your bike, and a stranger had come up and said "Aww nice bike man!" while casually touching the top tube, your reaction might not have been as severe. Treating your bike as just another piece of junk to lean against isn't quite the same.
Yeah actually I think that is what it was also ... I've seen people look at my bike when it's been locked up some place ... which is fine with me. He was just all leaning on it and gripping the saddle and stuff ... not cool. :< My poor bike! Oh well.