I am one of the people who do this. My reason is I am not belittling the person who yelled at me. When I do this, I am in the situation where I am wrong and they keep pointing my flaws over and over, ignoring the fact that I deeply regret about my mistakes. That makes me have no idea what to respond anymore. Deep inside I am very uncomfortable, like "I know that I am wrong but I have learned my mistakes and you don't have to be arrogantly rude to me". I want to yell back and smack their faces, but I know I'd get in the trouble by doing so. So smiling is the best way to fake it. Sometimes it is mixed with nervousness and trying to hide my fear too. So yeah, it is like what ThunderBow said.

But it is also possible like what's others said, maybe they are belittling you. I never done that though.