It is likely to be a combination of not knowing exactly how many crew hours they have available and some commercial negotiations that remain with several airports.
Remember that the 80 grounded aircraft last year were significantly (although not entirely) accounted for by the face the pilots did not have enough hours to operate the fleet through the winter. This year the problem is, if anything, a little more acute. RYR will do anything (to their credit) to avoind cancelling services due to crew shortages.
Wally.