Usually, when a stalker happens upon me (which may or may not be random, as we've seen), he comes up behind me and grabs me. The music and camera change and I mash X until Ezio throws him off.
Last night I was hanging out on an empty rooftop (so much for stalkers lurking in crowds, eh?) when a stalker showed up. Thing is, I didn't even know there was a stalker around, since Ezio had already wrecked the guy before I even knew he was there. He threw him off before the stalker managed to grab him. It was awesome, but I have no idea how it happened.
How'd I do that?
Best Answer
I found a couple of reports where people claimed stalkers on rooftops are just weirdly glitched. I think the game spawns them on the roof but then doesn't really know how to get them "stalking" properly. It wouldn't surprise me if encountering them on the roof occasionally triggered other glitches.
Apparently also turning around at the last moment causes Ezio to push them down instead of the stalker grabbing you (according to part of that thread which I apparently skimmed and did not fully process...), so maybe it's a combination of the two?