zett - 6:45 am on Mar 30, 2012 (gmt 0)
Since you specifically asked for speculation, here's my best guess: Ad networks have lots of SEM specialists who probably issue lower bids than your typical B&M business that does not know too much about placing ads on Adsense. By unblocking those ad networks, you open your site to those cheaper ads (from the pro's), probably pushing away the B&M ads. In addition, those ad networks might get better placements due to volume.
No, it's no surprise to me that your average EPC goes down.
(Disclaimer: I have stopped actively managing Adsense long ago and removed most ads from our sites. It just was not worth the hazzle any longer.)