There is some buzz around the community that there may have been a Google update, specifically a Penguin update, yesterday. It really seems to all stem from this post by Accuranker which shows a spike in their tracking tools.
But I can say, the SEO community seems too quiet, at least this morning, for there to be a Penguin update going on.
The ongoing WebmasterWorld forum is dead silent, there is a thread at BlackHatWorld and DigitalPoint Forums referencing the Accuranker post but again, no one is saying they are seeing it with their rankings.
The interesting part is that Accuranker is not the only one seeing this change, so is RankRanger, SERP Metrics and there is a bit of a spike on Mozcast but not as big as you'd see with Penguin. SERPs.com, Algoroo and Advanced Web Ranking all show normal activity. Update, Mozcast is now showing even higher temperatures.
So my guess, maybe Google was testing something but I don't think there is a Penguin update - yet.
Update from Mozcast:
Temps yesterday and In-depth Article count (for 2 days) are wrong. Should have updated data tomorrow (due to some Google changes).— MozCast (@mozcast) January 6, 2016
This makes sense why some tools spiked and some did not.