Why does brake fade tend to get progressively worse the longer a driver keeps riding the brakes on a downgrade?

49 CFR 383.111(b)(7)(i) (Proper braking techniques); AAMVA CDL Manual, Section 5, subsection 5.4.5 (Brake Fading or Failure), p. 5-9

Understanding the heat-expansion-weaker-braking cycle explains why fade is self-reinforcing: once it starts, continuing to ride the brakes only makes stopping power worse, not better. That is why the only reliable fix is to stop riding the brakes and let them recover, using the engine and the apply-release cycle instead.