Don't understand it either and its becoming more and more common and therefore more and more irritating (to me anyway), maybe not anti-social but definitely irritating. Do they not realise I've no interest in whatever dribble they're spouting loudly to each other.
I don't understand why so many people flock to the likes of starbucks / costa etc when there are plenty of generally better independent coffee / tea shops much more deserving of peoples custom.