Can be a combination of things:
- grit penetrationg through to the metal
- road goo (chemicals, salts, etc.) getting through to the metal and initiating corrosion
- staining
If you check, you will find most older bikes all have this. One solution is to take the lowers off and polish them (buff with a wheel, grinder, or drill), or there are bunches of products supposed to address this. Just depends on your personal preference.
TQ