This was probably one of the Google's "Panda Refreshes", which happens almost every month, to keep a check on websites that spam and are of low quality(Learn to make Google friendly sites).There have been several of these refreshes in the last few months.
These "Panda Refreshes" give bloggers and webmasters a chance to recover from the panda setbacks.
Even Seoroundtable referred to this Panda Update and they say that Google has denied that they had made a big change, but us webmasters think otherwise.
If you are concerned that your blog has been affected by the Panda updates, then read - Check For Panda Penalty
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