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