That's basically the intention, to offend them. It's a gesture of insult/disrespect. It's the hand sign for "f*ck you," so they get mad for the same reason they do when you say that.

To go a little deeper, I suspect that the reason people get mad back at you when you get mad at them is because their first reaction would be to feel bad/grief that they apparently did something wrong to make you feel that way, because on a superconscious level we all really love each other. But we feel we shouldn't have to feel that grief, because people often get mad at other people for reasons that aren't their fault, or we just don't want to have to feel the grief, so we get angry instead at the person for making us/trying to make us feel that way.