This is called fretting corrosion. It's caused when the screws abd rivets become just ^slighty looser than spec. You wouldn't be able to tell at all in the beginning stages, and if you felt those they'd likely feel perfectly secure.

But they are loose enough for microscopic particles to get between the fastener head and the material, and when they get there they slowly grind material away that escapes out from under the head, and streaks backwards in the wind.

These are also called smoking rivets.