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!
The difficulty bomb will start to phase in in November. So I think the Ethereum developers has about 1-2 month to solve the problem.