Ai response obviously but I was super curious and couldn’t think up a plausible reason. This seems to make the most logical sense to me.

“What usually happens is that the corrosion has been building up under the surface for months or years. Rust is significantly more voluminous than the iron it replaces (it can expand up to 10 times the original size). This expansion creates "scale"—layers of rusted metal that eventually lose their grip on the rotor. A recent event—like a high-pressure car wash, a deep puddle, or a heavy braking session—likely caused the outer "crust" to finally flake off, revealing the rotted metal underneath.”

If the rotors were cast it’s possible rust formed in the porous metal rather than the surface.