Making an educated guess as a layperson, besides some satellites that are geosynchronous but not geostationary, I'd assume those are primarily old geostationary satellites in graveyard orbits - when they're EOL, satellites in those orbits are supposed to perform a small boost out of them, usually adding a few hundred km to their orbit's radius (GEO is about 36,000km in altitude, so a few hundred km is relatively small). Then, without station keeping, I believe they should naturally precess around the Laplace plane, which will range between roughly Earth's equator and the ecliptic plane (the plane of Earth's orbit). At GEO altitudes the Laplace plane is about 7.2 degrees inclined from the equator. I believe that would mean, starting at the equator with an inclination of 0 degrees, these satellites should precess to about 14.4 degrees and back to 0 over several decades (excluding other perturbations, of course).

I found this online which would seem to confirm at least the mechanics: https://amostech.com/TechnicalPapers/2013/Orbital_Debris/ROSENGREN.pdf

From what I could find that’s probably the band of decommissioned geosynchronous satellites. Apparently they’ll slowly match the orbital plane of the earth around the sun.

https://space.stackexchange.com/questions/12528/why-is-the-ribbon-of-decommissioned-geosynchronous-satellites-skewed

those are mostly red and orange, so that also confirms the other comments' guesses: