plc90210 - 11:35 pm on Mar 17, 2013 (gmt 0)
"It's possible also that the anchor text didn't exhibit natural variation"
The anchor text of most of the links was the name of the site/domain - don't see how that's unnatural.
"that the rate of growth was too fast"
Matt Cutts has said that the rate of link acquisition/link velocity is largely irrelevant to rankings: [youtube.com...]
"that the sources may have been in some way related, either to each other or to you"
"Perhaps Google didn't see enough traffic to the site to warrant the links."
Google doesn't consider traffic/analytics data for rankings: [youtube.com...]
"I'm assuming that Google didn't see these as freely given, relevant, editorial links."
Most of the links were from news/blogs sites in my niche. One was from a city government site's page about my niche (PR 4).