The rubber consists of an enormous tangle of long chains of molecules in the form of molecular spaghetti with one extra complication. The molecules of rubber consist of intertwined and cross-linked long chains of atoms in roughly random orientation.
When we pull out such a tangle, some of the chains tend to line up along the direction of pull. At the same time, the chains are in thermal motion, so they hit each other continually. It follows that such a chain, if stretched, would not by itself remain stretched, because it would be hit from the sides by the other chains and the other molecules, and would tend to kink up again. So when one pulls the rubber band out, the chains are length wise, and the thermal agitations of the molecules on the sides of the chains tend to kink the chains up, and make them shorten.
If the chains are held stretched and the temperature is increased, so that the vigor of the bombardment on the sides of the chains is also increased, the chains tend to pull in, and they are able to pull a stronger weight when heated. If, after being stretched for a time, a rubber band is allowed to relax, each chain becomes soft, and the molecules striking it lose energy as they pound into the relaxing chain. So the temperature falls