And another example was proven earlier when they dropped the rankings of a number of sites they thought were “parked domains.” It happens.
Basically, google is constantly changing their algorithm and finding poor results from them. So, if you do notice problems in your rankings for unjustified reasons, make sure you complain across the proper channels despite how little we think they listen. Tweet back to Matt Cutts. His Google Plus account is becoming more active and maybe you can comment on what he says there. He actually even commented on this particular subject recently:
I saw a recent post where several sites were asking about their search rankings. The short explanation is that it turns out that our classifier for parked domains was reading from a couple files which mistakenly were empty. As a result, we classified some sites as parked when they weren’t.
I apologize for this; it looks like the issue is fixed now, and we’ll look into how to prevent this from happening again.
But more details on the parked domain issues can be further explained in the following posts by Danny Sullivan and his initial post describing Matt’s comments above.
Lastly, one quick note, but if you want to see what Matt Cutts is posting regularly on his Google Plus account, just click here. 
