What happened in 2000?

The crash.

Maybe people were searching the term more when it was unfamiliar for them, looking for a definition, and then it fell off as most people interested became aware of what it means?

Very interesting! Perhaps around that time was the "tipping point" when clipless went from being an option to the norm? So it would have been a hot topic then. Just a theory.

The graph shows usage of the terms in a corpus of about 5m books indexed by Google. Nothing to do with searching.

Also there's a very small bump around 1970 which is odd - could be skis, maybe?

Lance Armstrong

This.

How does "the Lance effect" account for the falloff after 2000? Wouldn't you expect to see an increase between 1999 and 2005 if it was Lance-related?

The uptick is usage in books, and it starts in the late 1980s.

Given the subject matter, I believe this might be an apropos response to your post:

wut?

... which is about 4 % of all the books that have ever been written.