I believe it's due to the point of ascension changing. With the earth tilting at 23.5 degrees, a GEO orbit also has to be at that angle relative to the sun. Onc

I believe it's due to the point of ascension changing. With the earth tilting at 23.5 degrees, a GEO orbit also has to be at that angle relative to the sun. Once they are no longer operational, they will stay at that 23.5 degree angle, but the point of ascension will drift as the altitude (and therefore the orbital period) changes.