Assuming this is even taking place, which Im not convinced off, a large mining entity like ghash might have several incentives to do this; a block witholding attack  would hurt their smaller competitors for almost no cost, possibly driving miners away from these "unlucky" pools towards their own,  and it would keep difficulty artificially low. Once you get big enough,  adding more hashrate doesnt bring you a lot of extra mining revenue, it mostly increases difficulty and therefore your own cost as you start competing with yourself. Then its probably better to hide your hashrate this way, its not adding to difficulty, yet you are still getting paid for it. Maybe organofconti can do the math at what point this makes sense. But this would also explain some of the huge swings we see in the estimated network hashrate, which seem to be too big to be caused by chance.