Robert_Charlton - 11:04 pm on Mar 17, 2013 (gmt 0)
Sounds offhand like you built links to the homepage in ways that didn't appear natural for Google.
It's possible also that the anchor text didn't exhibit natural variation, that the rate of growth was too fast, and 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. Whatever, I'm assuming that Google didn't see these as freely given, relevant, editorial links.
That's a quick guess, without knowing anything else about the situation.