Both. Tannins rise to the surface and oils yellow.

Yes, but does oil stain do the same?

Yes and no. All coatings are affected by electromagnetic and UV radiation. In fact, almost everything is, even diamonds lose atoms in sunlight.

Oil based coatings tend to amber and water based tend to fade, but oil based can fade and water based can amber.

The biggest culprit in yellowing is the crosslinker, which create hardness by helping coatings bind to themselves, but binders also amber over time. Since stains have binders, they will amber, but usually quite a bit slower than paints do. Some colors and coating are slow enough to be considered color-fast. The more penetrating a stain is, the less prone to color change it is. Stains have pigments and/or dyes. More pigment means a thicker film, which means more color change, however, pure dye and mostly dye stains will amber slow enough to be considered non-yellowing. Anything that lightens wood will be the worst, so whitewashes and gray tints generally yellow the fastest, next is really dark colors. Really anything that obscures the grain will be more susceptible to color change.

Thanks. This info helps!

Wood will change color regardless of finish. The finish just slows the color change.