That must be their way of saying hello.
Yeah, like somebody's dog jumping up and biting you, and the owner explains it away with, "he can't wag his tail so he has to bite you, ha ha."
I encountered the orc hunters outside of Riften, opposite the stable house. I wasn't sure who they were at first and so approached. "That's far enough," one of them said, as well as the other usual warnings. I don't remember exactly what happened after that, but I vaguely recall getting a bounty for killing one of them.