My guess is that water and road salt were getting inside the seat tube and pooling up in the bottom bracket shell. With no way to drain out, the water just sat inside of the steel frame and caused all of that rust. Here's what my three year old bottom bracket looks like now:
Might as well have pulled that out of a lake. I used a wire wheel on a drill to scrape off as much rust as possible, then painted the inside of the shell and the bottom bracket itself. Finally, to make sure this doesn't happen again, I drilled a small hole at the bottom of the shell so that water has a chance to escape rather than cause all this corrosion.
This is worth doing if you ride in bad weather, since it can protect your frame and bottom bracket from rust.
