All ships must go in periodically for a complete check up of all systems. That is mandatory. As the Oasis was built in Europe and they have the only dry dock that will take her, and they built her there in the first place, arrangements have been made to sail her over and do a series of 5 days cruises in Europe then a 7 day cruise to the dry dock. From there they will sail a transatlantic back to the U.S.
On a personal note, I think they also want to try sailing an Oasis class in Europe and see how well it will be received as they consider moving one over there when the third Oasis class comes out.