Why does a polymer become more flexible above its glass transition temperature?

A polymer becomes more flexible above its glass transition temperature ($T_g$) because of a fundamental change in molecular motion.

Below $T_g$, a polymer exists in a glassy state, where the molecules have low energy and are essentially frozen in place, resulting in a stiff and brittle material. As the temperature rises to $T_g$, the molecules gain sufficient energy to vibrate more vigorously.12

Once the temperature exceeds $T_g$, the polymer enters a rubbery state. In this state, the increased molecular motion allows the polymer chains to move past one another more easily. This enhanced mobility provides the material with greater flexibility, elasticity, and the ability to be stretched or molded. Essentially, the transition marks the point where the internal energy overcomes the forces that previously held the molecular structure in a rigid, fixed arrangement.123