Most probably this has to do with the World Cup. Maybe Qatar wants to have a third hotel ship in addition to Poesia and Europa. With the end of december cruises you should be save. The Europa leaves Doha on Dec 20, the the Opera is bookable already from Dec 18 for a cruise without Doha.
Sure, Qatar took already two MSC ships. But only last week they found out that they still lack of accomodation and announced that no tourists without World Cup tickets can enter in November and December. Grabbing a third MSC ship is probably the easiest solution. But thats still just a guess.