I’m running a german surfwebsite (www.wellenreiten.de) and used Google Trends a lot for research. Two things to consider:

1) “Surfing” is used in other connotations, especially “surfing the internet”. That should have declined over the years like the use of the words “portal” or “multimedia”. Check searches for “surfboard” instead and the picture looks different.

2) Regarding Quik and BBG: The surf theme has been picked up by a lot of other retailers, namely Hollister, stealing the show. So, yes, it became mainstream but unfortunately not like those two companies imagined.

Regards
Joerg