Why would they do this? I've got a theory: to pick up some cheap BTC. Actually nice way to manipulate the markets. They have to be considered a hostile entity now. Imagine the same with floating fees.
the number of double spends within the last 500,000 transactions actually went down. Just because someone has a lot of hashingpower does not mean the instantly use to double spend.
https://blockchain.info/double-spendsI would advice everyone holding GH/s @ cex.io and using GHash.io as a pool to switch anyway. Just in Case.