PET is highly hygroscopic and absorbs moisture from the atmosphere. Small amount of moisture will hydrolyze PET in the melt phase, reducing molecular weight. PET must be dried just prior to processing and amorphous PET requires cristallizing prior to drying so that the particles dont stick together as they go though glass transition.
Hydrolysis can occur due to moisture and this often can be seen as a reducrion in the Intrinsic Viscosity( IV) of the product. PET is semi-crystalline. When the IV is reduced, the bottles are more brittle.
In its crystalline state it has both crystalline and amorphous portions in its molecular structure. The crystalline portion develops where the molecules can align themselves in a very compact linear structure. In the non-crystalline regions the molecules are in a more random arrangement. By insuring that your crystallinity is high, prior to processing, the result will be a more uniform and higher quality product.
Another thing to consider is the number if times the PET has been processed. Each time the PET is processed there is a reduction in IV. therefore PET that has been used to make a bottle recycled and used again, does not have the IV of the original bottle. Each time the bottle is recycled the IV is further reduced. This is why a percentage of virgin resin is often added to increase the products properties.