I am not sure if this is just a coincidence, nor that it is appropriate on this site. What do you think?
3Happens all the time on SO, unfortunately.– Nick ChammasFeb 21, 2012 at 15:54
None of those answers have downvotes on them now so perhaps the system picked up the suspicious voting pattern and rolled them back?
You are right, behaviour like that is completely inappropriate. Even mods cannot find out who placed a vote (short of looking at every user's rep history) but if anything like this happens again let us know and we'll try and get the user suspended through the network.