Or, we're seeing the unintended result of several combined factors. That's what happened at first with the "sandbox" phenomenon: webmasters noticed it before Google engineers could understand what we were seeing.
This is my guess. While I can subscribe to a -950 penalty of sorts (basically being pushed to the end of results), I don't think Google would do anything specific to move sites down exactly five positions.
Based on a recent increase in my own traffic think that Google is tweaking something in their algo that might be displacing some long-time position holders for certain phrases. The consequence of that tweak is that long-time holders of a #1 position are suddenly moving down several positions. As mentioned in another thread, this could be related to this quote:
Now we're coming to the next major milestone in the elimination of the artificial difference between indices: rather than searching some part of our index in more depth for obscure queries, we're now searching the whole index for every query.