Where does disc rotting start?

[TL7]

After the lifespan of a disc is hitting end, small spots on the disc lose opacity.
But where exactly does it start? What does determine the first pin-prickle spot?

[Wombler]

Variations in the quality of manufacture across the individual disc.

Even a pristine premium brand disc can never be perfectly uniform and the poorest areas will go first.

Physical damage to the disc can also potentially impact on things and by damage I mean everything from scratches to chemical damage such as ink from a pen used to label the disc.