Not sure if the truth is just too dull for you, but whoever's building the home page for the next day goes through active conversations during the day - picks a couple of interesting stories, usually with as wide appeal as possible, or that have humour, are unusual etc etc - adds them and deletes the two bottom discussions....
Would love to say we had some clever strategy, but really it's whatever catches the eye/imagination of the person on duty.... there you go the whacy world of mumsnet HQ