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.
The difficulty is still rise very fast at the moment. It is rising 25% every months on average. So the miners think the PoW will continue.