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?