Why do some people cough all the time for seemingly no reason?

I know what you're talking about OP. Some people do it  ("cough cough") to draw attention to themselves or to annoy or mock someone else. It's an immature thing to do in the latter context, like an alternative way of laughing at someone. Immature people do it; I notice teenagers do it a lot. (No offense to people who cough for health related reasons, that's not what the OP is on about.)