Why do rock layers bend?？

Anticlines and synclines are two basic forms of folds; if folds are classified based on the relative age and morphological characteristics of the strata that form them, they can be divided into two basic types—anticlines and synclines.

Syncline: Meaning downward inclination, refers to folds where both limbs extend upward, and the center bends downward. The core consists of younger strata, and the limbs consist of older strata.

Anticline: Generally appears as an upwardly convex bend. The strata tilt outward from the center, with the core consisting of older strata and the limbs of younger strata.