oxidation of cellulose and uv degradation. stick paper in light proof vacuum, it will always be white. Possibly some other types of enzymatic browning.

I think acid plays also plays a role (hydrolyzing the cellulose?)- that is why there is "acid free paper".

There are some types of paper where the lignin has not been removed. One example is newsprint paper which consists partially of mechanical pulp. Under the influence of sunlight (UV) and oxygen the initially white color turns to brown or yellow gradually.

I believe the gradual change of paper form white to brown depends on the kind of atmosphere it is kept which can quite possibly lead to oxidation especially in a place where there is little oxygen, if paper is kept in a cool dry palce with lots of air, come back in 50 years the colour will still be the same precluding any atmospheric change.

There is one process descibed by Ludwig Gruber. An other process is the  gradual disintegration of so called blancophores which transform UV-light to blue-light. These blancophores are on all modern white papers. Natural paper is yellowish.