I'd guess it was done after a that's the way the show is. The strength of that is they can build up a kind of inevitable story line, the weakness that there are no surprises to the storylines that develop that could make the show more unpredictable as you might get in more 'live' shows like Big Brother.
Even so I still think they cast people to stir things up, and someone like Vanessa was probably encouraged in that direction.