This answer mentions several effects that tend to move geostationary satellites out of their "box" over time, but one really surprised me.
According to the plot below (from there, but uncredited) the semi-major axis of a satellite in GEO will climb by 21 kilometers in six months.
Is this really true? If so, could someone explain how this rapid orbit-raising is possible? I don't think there are tidal forces at work here, the spacecraft would not be massive enough to cause shape changes in the Earth big enough to do this.
If it is possible please include a sourced mathematical expression that reproduces approximately this rate of orbit-raising as well.
Graph 1 : Shows the drift of semi-major axis for a satellite placed at nominal R of 42164.2 km propagated for about 6 months shows an increase of about 21kms.
Source: [Book] Li, Hengnian. Geostationary satellites collocation. New York: Springer, 2014.