Why Are Chemical Bonds More Stable at Cooler Temperatures?

[DespicableMe]

Homework Statement

I was wondering why bonds are more stable at cooler temperatures than at higher temperatures.

This question came from a research project about annealing (heating then colling) plastics and why polyethylene maintains its shape when it's cooled.

 

[Borek]

What happens to atoms/molecules when temperature goes up?