I think it's a danger--and it happens often--that people who lose weight fast by exercising hours and hours while eating too little calories will gain back the weight if they stop exercising altogether and go back to their old ways with food.
But hopefully, the people on Biggest Loser have an "after-care plan" or maintenance plan to keep them from reverting like that. Probably it involves regular, but more moderate exercise, along with watching food intake and weighing regularly. Why, that sounds like Maintenance!
Jay