Theirs many reasons a chin will bark like that. The main reason is its a warning call. If they sence some kind of danger theyll sound off. Of course this means they could hear a noise outside, or just something their not frimalar with. And they will often do it a few times in a row. Sometimes they seem to do it for the heck of it to. Iv also seen mine personaly do it as a way to say 'quite down! im trying to sleep!'