Automatically arrange icons on the Mac OS 10.6 desktop in groups
Once the desktop fills up, finder will tile and stack icons with a very small offset. I'd guess the algorithm had a bug or your .DS_Store file was corrupt and the system thought it had more icons to pile than it ended up with.

I dont know of a way to intentionally trigger the effect, but it would be nice to have four or five tidy stacks nice and deep rather than using the whole desktop.