The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.
A cross-sectional view of the Van Allen radiation belts, noting the point where the South Atlantic Anomaly occurs
Intensity of the magnetic field in the center of the South Atlantic Anomaly, 1840 to 2020.
Area of the South Atlantic Anomaly, 1840 to 2020.
The highest intensity portion of the SAA drifts to the west at a speed of about 0.3 degrees per year, and is noticeable in the references listed below. The drift rate of the SAA is very close to the rotation differential between the Earth’s core and its surface, estimated to be between 0.3 and 0.5 degrees per year.
Current literature suggests that a slow weakening of the geomagnetic field is one of several causes for the changes in the borders of the SAA since its discovery. As the geomagnetic field continues to weaken, the inner Van Allen belt gets closer to the Earth, with a commensurate enlargement of the SAA at given altitudes.
This is what I told the physicists back in the early 1990’s as it all came to me in a vision and was explained to me.