I know there have been a few cases of this in the Secondary League since I've been here, not so sure about Primary League. To try and answer your question, and obviously DTM would be able to answer better than me, it's probably because tonight's event could affect your wagers for Saturday's event and cause some real headaches for a lot of people-let's say you've got a plan to wager $2500 on a certain fight Saturday but you lose some wagers tonight and find yourself without enough money for Saturday as a result. OK, maybe not the best example, just trying to think fast. The once Secondary case I can think of first is when the events were on consecutive days or even the same day-on the second event nobody could make wagers because of the time constraints but everyone still got prediction bonuses.
Alright, enough blabbering from me, someone who can better answer please do so.