I mean....I know that the difficulty going up means more ppl are mining, means better decentralization, etc. But FUCK! Mining profits keep getting smaller and smaller! 
It just mean the mining will be concentrated into cheap electricity places like China. It happened to the btc mining.
No, I doubt that's the main cause. POS is coming soon, at least, too soon to invest a giant mining operation into Eth. More likely, it's a combination of difficulty bomb (by design), and more rigs being brought online/switched to Eth mining.
Interesting.
From what I've read on other Threads - most of the difficulty rise comes from new mining hardware, improvements in GPU mining techniques and new people coming into crypto-mining because of ETH.
Someone could research and post figures on wallet-qt growth rate over the last 4 weeks!
If ETC FAIL, the miners on classic may want to hash on the ETH chain to make some coin. This means ETH Network hashrate could suddenly rise by 13% also increasing ETH difficulty by about the same. And vice-versa!
At moment: some threads are awash with people asking questions about how to set up 480's and 470's for mining Ethereum!
Some people are saying that 480's are sold out in there countries.