The snake in this case is the Snake River. The event started when one of the locals decided to try riding every crossing of the Snake River. Others thought it a grand idea, and the tradition has continued.
After 9/11, many of the crossings have been closed, as they are over dams. Nonetheless, the roads are great, the scenery excellent and the company sublime.
The first video covers the ride from Seattle, the second the the ride day itself, and the third is the ride home taking a liesurely 2 days on a longer route.