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Unfortunately, I've seen a lot of opinions on what happened and not a lot of solid evidence (including my own, which is inconclusive, sadly). Google tightened up on something content-related, it seems, but we're not sure what. It's hard to speak in generalarities, but if I had to start somewhere, I'd be looking for Panda-like signals. Do you have content that could be considered thin, and/or are you facing duplication either within your site or across competitors sites? Is your site generally too big, in terms of indexed pages, for your authority (i.e. the strength of your link profile)? It may simply be that Google turned up the volume on a few factors. If this is content-related, the good news is that fixes should generally impact your rankings fairly quickly. This doesn't seem to be a Penguin-like situation.

Beautiful, thank you!

Karma will Kome