system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some dogs kick their back leg when you scratch them in a certain place.

I mean does it annoy them? They can't seem to help it. 

EDIT:
Thanks. Great ideas.
gpt
It could be like a cat's. When you scratch a cat on the lower stomach and their hind legs go crazy scratching you, they don't do it on purpose. They actually do it to themselves sometimes. It's triggered because in the wild, when they would get pinned by the animal they're fighting, their hind legs would attack the vulnerable stomach of the other animal that is on top of them.