My 2 cents :

What you're describing reminds me something : sandbox
Or more exactly, a new kind of sandbox effect.

Last year, I observed something about Sandbox :
On august 2006 I decided to park my pool of domain names. I builded an home made parking system.
To my great surprise these crappy (about 20) websites hited top rankings... for a while.
In fact, they almost all folowed the same pattern :
1. Good start, high positions for a couple of month. Over performance.
2. Drop down in the dephs for 4-6 month
3. Slow up to "regular" rankings

My mind was that Google were using information life cycle in algo :
1. Artificial over exposure
2a. If over exposure create traffic and popularity => immediate good rankings
2b. If not, sandbox effect

Las month, I tried to proove this theory. But I lacked websites in my panel. It also seems that Google complexified (once again) its behavior.

These "systematic" supplemental results could be a new part of sandbox effect, don't you think ?