Not entirely certain.
I know that the late Xmas release dates have not translated well in terms of toy sales, hence the massive cuts in price of "Rogue One" stuff after a very short space of time in the shops. But the whole toy industry is kind of suffering at the moment with traditional toys taking a beating in this digital age.
Given Marvels heavy release schedule of films they may push one of these back a little into the Xmas slot.
But given we have now had the new Avatar movies release dates ,thats the likely reason, but they will not kick into the Xmas slots for quite a while and they are going to be spaced out in December 2020,2021,2024,2025. So maybe they are moving now to avoid a conflict of interest.
They are going to have to move like lighting to get IX done though and no reshoots. Thats a damn tight schedule.