This is an old behavior by Google SERPS. However, some light might be shed on this mystery through this 2007 patent filing by Google:
http://appft1.uspto.gov/netacgi/nph-...DN/20070094255
Read the part about spikes in backlinks and ranking resulting in "capping"



The good news is that there might be a time limit. Nice--Cutts, Matt... (credited as one of the inventors)