Does this happen to your aluminum Apple Watch?
It originates in the laser etched letters, that's why you'll see some letters falling out first and exposing the silver aluminum.
We suspect it's from aluminum dust that gets trapped inside the fine grooves of the letters. After laser etching, it's submerged in the anodizing bath. Since some of the fine aluminum dust particles weren't cleaned out completely, parts of the aluminum weren't properly sealed in the anodization process.
When you exercise or swim and get sweat, salt and other corrosive chemicals into these partially sealed letters, the salt/chemicals eat away at the un-anodized aluminum from the inside out, which is why the corrosion problem can get as bad as the picture you see here (although this is very extreme). If you've seen a few letters missing, replaced by raw silver aluminum, this is why.
And this is why it doesn't happen to any other areas of the watch.