Who is responsible for the paid link mess?

Look at this graph showing the number of [buy link] searches since 2004:

Interestingly this search term starts out in September or October 2004, and shows a quite stable trend until the recent paid links debate started.

Who or what caused SEOs to massively buy links since 2004?

• The Playboy interview with Google cofounders Larry Page and Sergey Brin just before Google was about to go public?
• Google's IPO?
• Rumors that Google ran out of index space and therefore might restrict the number of doorway pages in the search index?
• Nick Wilson preparing the launch of Threadwatch?
• AdWords and Overture no longer running gambling ads?
• The Internet Advancement scandal?
• Google's shortage of beer at the SES Google dance?
• A couple UK based SEOs invented bought organic rankings?

Seriously, buying links for rankings was an established practice way before 2004. If you know the answer, or if you've a somewhat plausible theory, leave it in the comments. I'm really curious. Thanks.