It is a combo of factors. They plan on bringing Mayo up and Holliday back up soon. Really the only places left for Mayo will be 1b or OF.
They have Mountcastle at 1b along with O’Hearn/ DH. Santander/ Hays, Mullins and Cowser in the OF.
They also have a bunch of studs at AA plus Bassalo.
So, apart from performance vs other team members, age, contracts etc. They need to make room.
Unless of course they trade a bunch of their prospects. However, a guy like Mountcastle, they would probably not talk about putting on the block during a playoff run, unless they are confident in the internal solutions.