However, your response provokes yet another question. You can think of it as if the plastic is only part-way formed before exposed to the heat. Yet shrinkable tubing is clearly quite stable in its unshrunk form.
All plastics are at least somewhat polymerized.
And it's possible to make them all more polymerized. The more polymerized it is, the longer the chains are, and the more solid it will be. This is because the longer chains get more 'tangled,' so they stick together more tightly. So when you heat the tubing up, the chains get even longer, get more tangled and closer together, and the plastic takes up less space. An example of this is low and high density polypropelene.