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.
I think this is it. I'll sometimes be somewhere I can see my locked up bike from and I'll see people inspect it and touch it and sometimes turn the bars and such and it doesn't bother me in the least. I think if someone just leaned on it and rested his hands like it was his I'd get irritated