You really don't know whose hashrate that ist? It's easy: GHash.IO and/or CEX.io!
Why do you think their percentage of the network went down? Miners leaving? Nah! I believe they just relocated that hashrate to another address!
Hell, I think they've been obfuscating their blocks since the beginning of this year if not earlier. It wouldn't take a genius to figure out the mass panic that would ensue if that "
magical" 51% barrier is passed... Since ghash.io is making so much easy money off of their own ASICs as well as renting out all the mining power they can to suckers (even more profitable!) they wouldn't have much incentive to mess with the network, I'd think. Much better to just hide some blocks.