I was posting spun (paragraphs spins to 60% uniqueness and 20 spun titles) articles to a random selection of 40-75 of the blogs in the network. That spun content was the only thing linking the blogs together.
None of them had Webmaster Tools, Adsense, etc. installed. None of them were linked to each other. Nearly all of them were hosted on different IP addresses. The only 2 sites not to get de-indexed were 2 that had been hacked a few days earlier and so the content on those couldn't be found via a Google search.
My guess is that someone from Google found one of my sites and then manually or used a tool to Google search for sentences from various articles on that blog to find other blogs and then repeated the process.
I had a good run out of it (18 months at least) and made some alright money, but anyone not posting completely unique content on their blog networks should consider doing otherwise