It's either that or a sudden profound amount of businesses discovered they can bid on content placement separately from the search network. I always knew, if the majority ever found out about this ability, CTR would crash and burn due to lower quality results AND as a bonus, lower EPC too.
With the recent press about the arbitrage "problem" and "Google addressing the problem," it is very obvious to me some of your highest paying content clicks may have found the bid-by-content feature. A lot of businesses read Digg and Arbitrage was on the main page yesterday. Suddenly today, I see this thread which was started last night? I just put two and two together, but perhaps it is a coincidence? I personally don't believe in coincidences.
The upside? Once this blows over, new businesses will join and blindly pay $2 a click for <enter your target high paying phrase here> on both the search and content network again. Until then? Take this lesson and diversify.
Never, repeat, NEVER depend on one source as a means for most of your income. I learned this horrible lesson last year and I'm STILL recovering (IE piles of debt to get into new markets, fast..).