If it's happening between two males it's a dominance thing. They'll make quick circles around each other looking for advantage. Sometimes thats all you'll get and sometimes they'll start to lip lock. If one of them looks beat up you may want to remove him or the fish that did the beating to another tank.
If he's doing this in front of a female it is the begining of romance. :wink:
I've never noticed this with any of my female Mbuna but thats not to say they wouldn't.
unfortunately i don't know the sexes of a lot of my fish but usually the one doing it is my large yellow lab. do you think that means it is a male lab? i don't really see much lip locking or fighting in my tank so far. just random chasing but no one really seems hurt or upset by it