because the difficulty increase made it unprofitable for those miners, so they stopped.
Given the rise in BTC price, which is a greater percentage hike than the difficulty, that certainly isn't the cause.
I'm inclined to say it's a portion of the OCN miners that came in as a result of the forum thread there losing interested. Most the people on those boards are high school / college kids, and came here thinking Bitcoin was a way to make fast easy money with their gaming rigs. Once the thread got locked, it stopped getting promoted as much and now the surge is dying out. Just my personal theory for PART of the decline.
